Capella beacon block (#11566)

* in progress

* done, no tests yet

* fix ToBlinded()

* Revert "Auxiliary commit to revert individual files from 2e356b6f5b15d409ac15e825c744528591c13739"

This reverts commit 081ab74e88fb7d0e3f6a81e00fe5e89483b41f90.

* tests

* fix tests

* one more fix

* and one more

* review

* fix proto_test

* another fix

* do not return error when nil object is wrapped

* allow nil payload in body.Proto()

* correctly assert error

* nil checks in body.Execution()

* simplify PR

* Revert "Auxiliary commit to revert individual files from 5736c1f22f2d2f309b9303c13d0fb6b1679c6ecb"

This reverts commit 1ff3a4c864923f5c180aa015aa087a2814498b42.

* fix slice sizes in cloner tests

* better payload tests

* review

Co-authored-by: terencechain <terence@prysmaticlabs.com>

consensus-types/interfaces/beacon_block.go

@@ -24,6 +24,8 @@ type SignedBeaconBlock interface {
 	ToBlinded() (SignedBeaconBlock, error)
 	PbBellatrixBlock() (*ethpb.SignedBeaconBlockBellatrix, error)
 	PbBlindedBellatrixBlock() (*ethpb.SignedBlindedBeaconBlockBellatrix, error)
+	PbCapellaBlock() (*ethpb.SignedBeaconBlockCapella, error)
+	PbBlindedCapellaBlock() (*ethpb.SignedBlindedBeaconBlockCapella, error)
 	ssz.Marshaler
 	ssz.Unmarshaler
 	Version() int
@@ -66,6 +68,7 @@ type BeaconBlockBody interface {
 	HashTreeRoot() ([field_params.RootLength]byte, error)
 	Proto() (proto.Message, error)
 	Execution() (ExecutionData, error)
+	BLSToExecutionChanges() ([]*ethpb.SignedBLSToExecutionChange, error)
 }
 
 // ExecutionData represents execution layer information that is contained