Kiln mev boost (#3062)

## Issue Addressed

MEV boost compatibility

## Proposed Changes

See #2987

## Additional Info

This is blocked on the stabilization of a couple specs, [here](https://github.com/ethereum/beacon-APIs/pull/194) and [here](https://github.com/flashbots/mev-boost/pull/20).

Additional TODO's and outstanding questions

- [ ] MEV boost JWT Auth
- [ ] Will `builder_proposeBlindedBlock` return the revealed payload for the BN to propogate
- [ ] Should we remove `private-tx-proposals` flag and communicate BN <> VC with blinded blocks by default once these endpoints enter the beacon-API's repo? This simplifies merge transition logic. 

Co-authored-by: realbigsean <seananderson33@gmail.com>
Co-authored-by: realbigsean <sean@sigmaprime.io>

validator_client/src/signing_method.rs

@@ -33,9 +33,9 @@ pub enum Error {
 }
 
 /// Enumerates all messages that can be signed by a validator.
-pub enum SignableMessage<'a, T: EthSpec> {
+pub enum SignableMessage<'a, T: EthSpec, Payload: ExecPayload<T> = FullPayload<T>> {
     RandaoReveal(Epoch),
-    BeaconBlock(&'a BeaconBlock<T>),
+    BeaconBlock(&'a BeaconBlock<T, Payload>),
     AttestationData(&'a AttestationData),
     SignedAggregateAndProof(&'a AggregateAndProof<T>),
     SelectionProof(Slot),
@@ -47,7 +47,7 @@ pub enum SignableMessage<'a, T: EthSpec> {
     SignedContributionAndProof(&'a ContributionAndProof<T>),
 }
 
-impl<'a, T: EthSpec> SignableMessage<'a, T> {
+impl<'a, T: EthSpec, Payload: ExecPayload<T>> SignableMessage<'a, T, Payload> {
     /// Returns the `SignedRoot` for the contained message.
     ///
     /// The actual `SignedRoot` trait is not used since it also requires a `TreeHash` impl, which is
@@ -113,9 +113,9 @@ impl SigningContext {
 
 impl SigningMethod {
     /// Return the signature of `signable_message`, with respect to the `signing_context`.
-    pub async fn get_signature<T: EthSpec>(
+    pub async fn get_signature<T: EthSpec, Payload: ExecPayload<T>>(
         &self,
-        signable_message: SignableMessage<'_, T>,
+        signable_message: SignableMessage<'_, T, Payload>,
         signing_context: SigningContext,
         spec: &ChainSpec,
         executor: &TaskExecutor,