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/tests/block_verification.rs

@@ -351,6 +351,7 @@ async fn assert_invalid_signature(
             snapshots[block_index].beacon_block.canonical_root(),
             snapshots[block_index].beacon_block.clone(),
             NotifyExecutionLayer::Yes,
+            || Ok(()),
         )
         .await;
     assert!(
@@ -415,6 +416,7 @@ async fn invalid_signature_gossip_block() {
                         signed_block.canonical_root(),
                         Arc::new(signed_block),
                         NotifyExecutionLayer::Yes,
+                        || Ok(()),
                     )
                     .await,
                 Err(BlockError::InvalidSignature)
@@ -727,6 +729,7 @@ async fn block_gossip_verification() {
                 gossip_verified.block_root,
                 gossip_verified,
                 NotifyExecutionLayer::Yes,
+                || Ok(()),
             )
             .await
             .expect("should import valid gossip verified block");
@@ -923,11 +926,7 @@ async fn block_gossip_verification() {
     assert!(
         matches!(
             unwrap_err(harness.chain.verify_block_for_gossip(Arc::new(block.clone())).await),
-            BlockError::RepeatProposal {
-                proposer,
-                slot,
-            }
-            if proposer == other_proposer && slot == block.message().slot()
+            BlockError::BlockIsAlreadyKnown,
         ),
         "should register any valid signature against the proposer, even if the block failed later verification"
     );
@@ -956,11 +955,7 @@ async fn block_gossip_verification() {
                 .await
                 .err()
                 .expect("should error when processing known block"),
-            BlockError::RepeatProposal {
-                proposer,
-                slot,
-            }
-            if proposer == block.message().proposer_index() && slot == block.message().slot()
+            BlockError::BlockIsAlreadyKnown
         ),
         "the second proposal by this validator should be rejected"
     );
@@ -998,6 +993,7 @@ async fn verify_block_for_gossip_slashing_detection() {
             verified_block.block_root,
             verified_block,
             NotifyExecutionLayer::Yes,
+            || Ok(()),
         )
         .await
         .unwrap();
@@ -1037,6 +1033,7 @@ async fn verify_block_for_gossip_doppelganger_detection() {
             verified_block.block_root,
             verified_block,
             NotifyExecutionLayer::Yes,
+            || Ok(()),
         )
         .await
         .unwrap();
@@ -1184,6 +1181,7 @@ async fn add_base_block_to_altair_chain() {
                 base_block.canonical_root(),
                 Arc::new(base_block.clone()),
                 NotifyExecutionLayer::Yes,
+                || Ok(()),
             )
             .await
             .err()
@@ -1318,6 +1316,7 @@ async fn add_altair_block_to_base_chain() {
                 altair_block.canonical_root(),
                 Arc::new(altair_block.clone()),
                 NotifyExecutionLayer::Yes,
+                || Ok(()),
             )
             .await
             .err()