Use a cache of one entry to build attestation (#13300)

* Use a cache of one entry to build attestation

* Gazelle

* Enforce on RPC side

* Rm unused var

* Potuz feedback, dont use pointer

* Fix tests

* Init fetcher

* Add in-progress

* Add back missing lock

* Potuz feedback

* Update beacon-chain/rpc/prysm/v1alpha1/validator/attester_test.go

Co-authored-by: Potuz <potuz@prysmaticlabs.com>

---------

Co-authored-by: Potuz <potuz@prysmaticlabs.com>

beacon-chain/blockchain/testing/mock.go

@@ -73,6 +73,7 @@ type ChainService struct {
 	BlockSlot                   primitives.Slot
 	SyncingRoot                 [32]byte
 	Blobs                       []blocks.VerifiedROBlob
+	TargetRoot                  [32]byte
 }
 
 func (s *ChainService) Ancestor(ctx context.Context, root []byte, slot primitives.Slot) ([]byte, error) {
@@ -617,3 +618,8 @@ func (c *ChainService) ReceiveBlob(_ context.Context, b blocks.VerifiedROBlob) e
 	c.Blobs = append(c.Blobs, b)
 	return nil
 }
+
+// TargetRootForEpoch mocks the same method in the chain service
+func (c *ChainService) TargetRootForEpoch(_ [32]byte, _ primitives.Epoch) ([32]byte, error) {
+	return c.TargetRoot, nil
+}