Optionally skip RANDAO verification during block production (#3116)

## Proposed Changes

Allow Lighthouse to speculatively create blocks via the `/eth/v1/validators/blocks` endpoint by optionally skipping the RANDAO verification that we introduced in #2740. When `verify_randao=false` is passed as a query parameter the `randao_reveal` is not required to be present, and if present will only be lightly checked (must be a valid BLS sig). If `verify_randao` is omitted it defaults to true and Lighthouse behaves exactly as it did previously, hence this PR is backwards-compatible.

I'd like to get this change into `unstable` pretty soon as I've got 3 projects building on top of it:

- [`blockdreamer`](https://github.com/michaelsproul/blockdreamer), which mocks block production every slot in order to fingerprint clients
- analysis of Lighthouse's block packing _optimality_, which uses `blockdreamer` to extract interesting instances of the attestation packing problem
- analysis of Lighthouse's block packing _performance_ (as in speed) on the `tree-states` branch

## Additional Info

Having tested `blockdreamer` with Prysm, Nimbus and Teku I noticed that none of them verify the randao signature on `/eth/v1/validator/blocks`. I plan to open a PR to the `beacon-APIs` repo anyway so that this parameter can be standardised in case the other clients add RANDAO verification by default in future.

beacon_node/beacon_chain/src/test_utils.rs

@@ -2,7 +2,7 @@ pub use crate::persisted_beacon_chain::PersistedBeaconChain;
 pub use crate::{
     beacon_chain::{BEACON_CHAIN_DB_KEY, ETH1_CACHE_DB_KEY, FORK_CHOICE_DB_KEY, OP_POOL_DB_KEY},
     migrate::MigratorConfig,
-    BeaconChainError,
+    BeaconChainError, ProduceBlockVerification,
 };
 use crate::{
     builder::{BeaconChainBuilder, Witness},
@@ -604,7 +604,14 @@ where
 
         let (block, state) = self
             .chain
-            .produce_block_on_state(state, None, slot, randao_reveal, Some(graffiti))
+            .produce_block_on_state(
+                state,
+                None,
+                slot,
+                randao_reveal,
+                Some(graffiti),
+                ProduceBlockVerification::VerifyRandao,
+            )
             .unwrap();
 
         let signed_block = block.sign(
@@ -658,7 +665,14 @@ where
 
         let (block, state) = self
             .chain
-            .produce_block_on_state(state, None, slot, randao_reveal, Some(graffiti))
+            .produce_block_on_state(
+                state,
+                None,
+                slot,
+                randao_reveal,
+                Some(graffiti),
+                ProduceBlockVerification::VerifyRandao,
+            )
             .unwrap();
 
         let signed_block = block.sign(