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>

beacon_node/beacon_chain/src/test_utils.rs

@@ -733,6 +733,15 @@ where
         state.get_block_root(slot).unwrap() == state.get_block_root(slot - 1).unwrap()
     }
 
+    pub async fn make_blinded_block(
+        &self,
+        state: BeaconState<E>,
+        slot: Slot,
+    ) -> (SignedBlindedBeaconBlock<E>, BeaconState<E>) {
+        let (unblinded, new_state) = self.make_block(state, slot).await;
+        (unblinded.into(), new_state)
+    }
+
     /// Returns a newly created block, signed by the proposer for the given slot.
     pub async fn make_block(
         &self,
@@ -1692,7 +1701,12 @@ where
         self.set_current_slot(slot);
         let block_hash: SignedBeaconBlockHash = self
             .chain
-            .process_block(block_root, Arc::new(block), NotifyExecutionLayer::Yes)
+            .process_block(
+                block_root,
+                Arc::new(block),
+                NotifyExecutionLayer::Yes,
+                || Ok(()),
+            )
             .await?
             .into();
         self.chain.recompute_head_at_current_slot().await;
@@ -1709,6 +1723,7 @@ where
                 block.canonical_root(),
                 Arc::new(block),
                 NotifyExecutionLayer::Yes,
+                || Ok(()),
             )
             .await?
             .into();