[Feature] [cosmos_node_client] Transaction signing

[andrzejchm]

Acceptance Criteria

• introduce new types CosmosTransaction and SignedCosmosTransaction:

class CosmosTransaction {
   final List<GeneratedMessage> messages; //comes from the google's protobuf https://pub.dev/documentation/protobuf/latest/protobuf/GeneratedMessage-class.html
   final String? memo;
   final Fee? fee; //comes from the proto/cosmos/tx protobuf found here: https://github.com/cosmos/cosmos-sdk/blob/master/proto/cosmos/tx/v1beta1/tx.proto
   final 

  Future<Either<TransactionSigningFailure, SignedCosmosTransaction> sign(Account account);
  // should mimic the functionality of https://github.com/alan-sdk/alan.dart/blob/604bc3484dafcc9c7634cce088ec8f8c09c96a70/lib/transactions/signer/tx_signer.dart#L41
}

class SignedCosmosTransaction {
    final Tx tx;
}

[Zfinix]

Starting work on this