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.

common/eth2/src/lib.rs

@@ -1155,6 +1155,18 @@ impl BeaconNodeHttpClient {
         slot: Slot,
         randao_reveal: &SignatureBytes,
         graffiti: Option<&Graffiti>,
+    ) -> Result<ForkVersionedResponse<BeaconBlock<T>>, Error> {
+        self.get_validator_blocks_with_verify_randao(slot, Some(randao_reveal), graffiti, None)
+            .await
+    }
+
+    /// `GET v2/validator/blocks/{slot}`
+    pub async fn get_validator_blocks_with_verify_randao<T: EthSpec>(
+        &self,
+        slot: Slot,
+        randao_reveal: Option<&SignatureBytes>,
+        graffiti: Option<&Graffiti>,
+        verify_randao: Option<bool>,
     ) -> Result<ForkVersionedResponse<BeaconBlock<T>>, Error> {
         let mut path = self.eth_path(V2)?;
 
@@ -1164,14 +1176,21 @@ impl BeaconNodeHttpClient {
             .push("blocks")
             .push(&slot.to_string());
 
-        path.query_pairs_mut()
-            .append_pair("randao_reveal", &randao_reveal.to_string());
+        if let Some(randao_reveal) = randao_reveal {
+            path.query_pairs_mut()
+                .append_pair("randao_reveal", &randao_reveal.to_string());
+        }
 
         if let Some(graffiti) = graffiti {
             path.query_pairs_mut()
                 .append_pair("graffiti", &graffiti.to_string());
         }
 
+        if let Some(verify_randao) = verify_randao {
+            path.query_pairs_mut()
+                .append_pair("verify_randao", &verify_randao.to_string());
+        }
+
         self.get(path).await
     }
 

common/eth2/src/types.rs

@@ -627,8 +627,14 @@ pub struct ProposerData {
 
 #[derive(Clone, Serialize, Deserialize)]
 pub struct ValidatorBlocksQuery {
-    pub randao_reveal: SignatureBytes,
+    pub randao_reveal: Option<SignatureBytes>,
     pub graffiti: Option<Graffiti>,
+    #[serde(default = "default_verify_randao")]
+    pub verify_randao: bool,
+}
+
+fn default_verify_randao() -> bool {
+    true
 }
 
 #[derive(Clone, Serialize, Deserialize)]