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/beacon_chain.rs

@@ -2578,6 +2578,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                         signature_verified_block.block_root(),
                         signature_verified_block,
                         notify_execution_layer,
+                        || Ok(()),
                     )
                     .await
                 {
@@ -2666,6 +2667,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         block_root: Hash256,
         unverified_block: B,
         notify_execution_layer: NotifyExecutionLayer,
+        publish_fn: impl FnOnce() -> Result<(), BlockError<T::EthSpec>> + Send + 'static,
     ) -> Result<Hash256, BlockError<T::EthSpec>> {
         // Start the Prometheus timer.
         let _full_timer = metrics::start_timer(&metrics::BLOCK_PROCESSING_TIMES);
@@ -2684,6 +2686,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                 &chain,
                 notify_execution_layer,
             )?;
+            publish_fn()?;
             chain
                 .import_execution_pending_block(execution_pending)
                 .await
@@ -2725,7 +2728,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             }
             // The block failed verification.
             Err(other) => {
-                trace!(
+                debug!(
                     self.log,
                     "Beacon block rejected";
                     "reason" => other.to_string(),