addSignersToTransactionMessage should upgrade feePayer to a signer

[steveluscher]

Motivation

Imagine a transaction message with a non-signer fee payer, having address 'abc'. Imagine a signer also having address 'abc'.

If you call addSignersToTransactionMessage() on that message, it should notice that the fee payer address matches the signer address, and should ‘upgrade’ the feePayer to a signer.

Example use case

const message = pipe(
      createTransactionMessage({ version: 0 }),
      m => setTransactionMessageFeePayer(signer.address, m), // non-signer version of this function
      m => addSignersToTransactionMessage([signer], m),
);
message.feePayer satisfies TransactionSigner<'abc'>;

[lorisleiva]

Thanks! I'll tackle this as soon as I can.