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/lib.rs

@@ -41,8 +41,8 @@ mod validator_pubkey_cache;
 
 pub use self::beacon_chain::{
     AttestationProcessingOutcome, BeaconChain, BeaconChainTypes, BeaconStore, ChainSegmentResult,
-    ForkChoiceError, HeadInfo, HeadSafetyStatus, StateSkipConfig, WhenSlotSkipped,
-    INVALID_JUSTIFIED_PAYLOAD_SHUTDOWN_REASON, MAXIMUM_GOSSIP_CLOCK_DISPARITY,
+    ForkChoiceError, HeadInfo, HeadSafetyStatus, ProduceBlockVerification, StateSkipConfig,
+    WhenSlotSkipped, INVALID_JUSTIFIED_PAYLOAD_SHUTDOWN_REASON, MAXIMUM_GOSSIP_CLOCK_DISPARITY,
 };
 pub use self::beacon_snapshot::BeaconSnapshot;
 pub use self::chain_config::ChainConfig;