Add broadcast validation routes to Beacon Node HTTP API (#4316)

## Issue Addressed

 - #4293 
 - #4264 

## Proposed Changes

*Changes largely follow those suggested in the main issue*.

 - Add new routes to HTTP API
   - `post_beacon_blocks_v2`
   - `post_blinded_beacon_blocks_v2`
 - Add new routes to `BeaconNodeHttpClient`
   - `post_beacon_blocks_v2`
   - `post_blinded_beacon_blocks_v2`
 - Define new Eth2 common types
   - `BroadcastValidation`, enum representing the level of validation to apply to blocks prior to broadcast
   - `BroadcastValidationQuery`, the corresponding HTTP query string type for the above type
 - ~~Define `_checked` variants of both `publish_block` and `publish_blinded_block` that enforce a validation level at a type level~~
 - Add interactive tests to the `bn_http_api_tests` test target covering each validation level (to their own test module, `broadcast_validation_tests`)
   - `beacon/blocks`
       - `broadcast_validation=gossip`
         - Invalid (400)
         - Full Pass (200)
         - Partial Pass (202)
        - `broadcast_validation=consensus`
          - Invalid (400)
          - Only gossip (400)
          - Only consensus pass (i.e., equivocates) (200)
          - Full pass (200)
        - `broadcast_validation=consensus_and_equivocation`
          - Invalid (400)
          - Invalid due to early equivocation (400)
          - Only gossip (400)
          - Only consensus (400)
          - Pass (200)
   - `beacon/blinded_blocks`
       - `broadcast_validation=gossip`
         - Invalid (400)
         - Full Pass (200)
         - Partial Pass (202)
        - `broadcast_validation=consensus`
          - Invalid (400)
          - Only gossip (400)
          - ~~Only consensus pass (i.e., equivocates) (200)~~
          - Full pass (200)
        - `broadcast_validation=consensus_and_equivocation`
          - Invalid (400)
          - Invalid due to early equivocation (400)
          - Only gossip (400)
          - Only consensus (400)
          - Pass (200)
 - Add a new trait, `IntoGossipVerifiedBlock`, which allows type-level guarantees to be made as to gossip validity
 - Modify the structure of the `ObservedBlockProducers` cache from a `(slot, validator_index)` mapping to a `((slot, validator_index), block_root)` mapping
 - Modify `ObservedBlockProducers::proposer_has_been_observed` to return a `SeenBlock` rather than a boolean on success
 - Punish gossip peer (low) for submitting equivocating blocks
 - Rename `BlockError::SlashablePublish` to `BlockError::SlashableProposal`

## Additional Info

This PR contains changes that directly modify how blocks are verified within the client. For more context, consult [comments in-thread](https://github.com/sigp/lighthouse/pull/4316#discussion_r1234724202).


Co-authored-by: Michael Sproul <michael@sigmaprime.io>

common/eth2/src/lib.rs

@@ -322,6 +322,26 @@ impl BeaconNodeHttpClient {
         ok_or_error(response).await
     }
 
+    /// Generic POST function supporting arbitrary responses and timeouts.
+    async fn post_generic_with_consensus_version<T: Serialize, U: IntoUrl>(
+        &self,
+        url: U,
+        body: &T,
+        timeout: Option<Duration>,
+        fork: ForkName,
+    ) -> Result<Response, Error> {
+        let mut builder = self.client.post(url);
+        if let Some(timeout) = timeout {
+            builder = builder.timeout(timeout);
+        }
+        let response = builder
+            .header(CONSENSUS_VERSION_HEADER, fork.to_string())
+            .json(body)
+            .send()
+            .await?;
+        ok_or_error(response).await
+    }
+
     /// `GET beacon/genesis`
     ///
     /// ## Errors
@@ -654,6 +674,76 @@ impl BeaconNodeHttpClient {
         Ok(())
     }
 
+    pub fn post_beacon_blocks_v2_path(
+        &self,
+        validation_level: Option<BroadcastValidation>,
+    ) -> Result<Url, Error> {
+        let mut path = self.eth_path(V2)?;
+        path.path_segments_mut()
+            .map_err(|_| Error::InvalidUrl(self.server.clone()))?
+            .extend(&["beacon", "blocks"]);
+
+        path.set_query(
+            validation_level
+                .map(|v| format!("broadcast_validation={}", v))
+                .as_deref(),
+        );
+
+        Ok(path)
+    }
+
+    pub fn post_beacon_blinded_blocks_v2_path(
+        &self,
+        validation_level: Option<BroadcastValidation>,
+    ) -> Result<Url, Error> {
+        let mut path = self.eth_path(V2)?;
+        path.path_segments_mut()
+            .map_err(|_| Error::InvalidUrl(self.server.clone()))?
+            .extend(&["beacon", "blinded_blocks"]);
+
+        path.set_query(
+            validation_level
+                .map(|v| format!("broadcast_validation={}", v))
+                .as_deref(),
+        );
+
+        Ok(path)
+    }
+
+    /// `POST v2/beacon/blocks`
+    pub async fn post_beacon_blocks_v2<T: EthSpec, Payload: AbstractExecPayload<T>>(
+        &self,
+        block: &SignedBeaconBlock<T, Payload>,
+        validation_level: Option<BroadcastValidation>,
+    ) -> Result<(), Error> {
+        self.post_generic_with_consensus_version(
+            self.post_beacon_blocks_v2_path(validation_level)?,
+            block,
+            Some(self.timeouts.proposal),
+            block.message().body().fork_name(),
+        )
+        .await?;
+
+        Ok(())
+    }
+
+    /// `POST v2/beacon/blinded_blocks`
+    pub async fn post_beacon_blinded_blocks_v2<T: EthSpec>(
+        &self,
+        block: &SignedBlindedBeaconBlock<T>,
+        validation_level: Option<BroadcastValidation>,
+    ) -> Result<(), Error> {
+        self.post_generic_with_consensus_version(
+            self.post_beacon_blinded_blocks_v2_path(validation_level)?,
+            block,
+            Some(self.timeouts.proposal),
+            block.message().body().fork_name(),
+        )
+        .await?;
+
+        Ok(())
+    }
+
     /// Path for `v2/beacon/blocks`
     pub fn get_beacon_blocks_path(&self, block_id: BlockId) -> Result<Url, Error> {
         let mut path = self.eth_path(V2)?;