privval: add grpc (#5725)

Co-authored-by: Anton Kaliaev <anton.kalyaev@gmail.com>

types/priv_validator.go

@@ -128,6 +128,11 @@ type ErroringMockPV struct {
 
 var ErroringMockPVErr = errors.New("erroringMockPV always returns an error")
 
+// Implements PrivValidator.
+func (pv *ErroringMockPV) GetPubKey() (crypto.PubKey, error) {
+	return nil, ErroringMockPVErr
+}
+
 // Implements PrivValidator.
 func (pv *ErroringMockPV) SignVote(chainID string, vote *tmproto.Vote) error {
 	return ErroringMockPVErr