Removes fork-specific concrete type getters (#13941)

* removing typed pb accessors

* refactor ssz api resp handlers to avoid typed pbs

* json get block handler refactor

* SubmitBlindedBlock to use generic json handling

* update SubmitBlindedBlock

* clear out more usages of PbForkname methods

* remove fork-specific getters from block interface

* remove usages of payload pb methods

* remove pb helpers from execution payload interface

* Update beacon-chain/rpc/eth/beacon/handlers.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* Update beacon-chain/rpc/eth/beacon/handlers.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* Update api/client/builder/client.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* Update api/client/builder/client.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* Update api/client/builder/client.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* Update api/client/builder/client.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* Update api/client/builder/client.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* Radek review

* fix error message

* deal with wonky builder responses

* ✂️

* gaz

* lint

* tweaks for deep source

---------

Co-authored-by: Kasey Kirkham <kasey@users.noreply.github.com>
Co-authored-by: Radosław Kapka <rkapka@wp.pl>

validator/client/propose.go

@@ -128,11 +128,16 @@ func (v *validator) ProposeBlock(ctx context.Context, slot primitives.Slot, pubK
 
 	var genericSignedBlock *ethpb.GenericSignedBeaconBlock
 	if blk.Version() >= version.Deneb && !blk.IsBlinded() {
-		denebBlock, err := blk.PbDenebBlock()
+		pb, err := blk.Proto()
 		if err != nil {
 			log.WithError(err).Error("Failed to get deneb block")
 			return
 		}
+		denebBlock, ok := pb.(*ethpb.SignedBeaconBlockDeneb)
+		if !ok {
+			log.WithError(err).Error("Failed to get deneb block - assertion failure")
+			return
+		}
 		genericSignedBlock = &ethpb.GenericSignedBeaconBlock{
 			Block: &ethpb.GenericSignedBeaconBlock_Deneb{
 				Deneb: &ethpb.SignedBeaconBlockContentsDeneb{