Implement skip_randao_verification and blinded block rewards API (#3540)

## Issue Addressed

https://github.com/ethereum/beacon-APIs/pull/222

## Proposed Changes

Update Lighthouse's randao verification API to match the `beacon-APIs` spec. We implemented the API before spec stabilisation, and it changed slightly in the course of review.

Rather than a flag `verify_randao` taking a boolean value, the new API uses a `skip_randao_verification` flag which takes no argument. The new spec also requires the randao reveal to be present and equal to the point-at-infinity when `skip_randao_verification` is set.

I've also updated the `POST /lighthouse/analysis/block_rewards` API to take blinded blocks as input, as the execution payload is irrelevant and we may want to assess blocks produced by builders.

## Additional Info

This is technically a breaking change, but seeing as I suspect I'm the only one using these parameters/APIs, I think we're OK to include this in a patch release.

crypto/bls/src/generic_signature.rs

@@ -80,6 +80,18 @@ where
         self.point.is_none()
     }
 
+    /// Initialize self to the point-at-infinity.
+    ///
+    /// In general `AggregateSignature::infinity` should be used in favour of this function.
+    pub fn infinity() -> Result<Self, Error> {
+        Self::deserialize(&INFINITY_SIGNATURE)
+    }
+
+    /// Returns `true` if `self` is equal to the point at infinity.
+    pub fn is_infinity(&self) -> bool {
+        self.is_infinity
+    }
+
     /// Returns a reference to the underlying BLS point.
     pub(crate) fn point(&self) -> Option<&Sig> {
         self.point.as_ref()