sign method
Implementation
Future<SignedBlock> sign(Signer signer) async {
final proto = toProto();
final typeUrl = 'dev.celest/${proto.info_.qualifiedMessageName}';
await signer.sign(utf8.encode(typeUrl));
final bytes = proto.writeToBuffer();
final signature = await signer.sign(bytes);
return SignedBlock(
block: bytes,
typeUrl: typeUrl,
signature: signature,
);
}