Add some syntax sugar to make it easy to use the new remote command runner API with particular generic command data specific to  the transactional context a RPC invocation is running under, like txnNumber and other metadata . Basically, eliminate the need for the helpers like the MultiStatementTransactionRequestSender under the new API, by adding syntax-sugar for async_rpc::send{Hedged}Command that automatically appends this transaction metadata. See https://github.com/mongodb/mongo/blob/master/src/mongo/s/multi_statement_transaction_requests_sender.cpp#L42-L59 for an example of how transaction metadata is added to requests under MSTRS - our new syntax sugar should essentially allow us to add the same data with the new API.