setTransactionMessageFeePayerSigner should overwrite signer when different

[steveluscher]

Presently, setTransactionMessageFeePayerSigner() doesn't let someone change the TransactionSigner if one already exists. If I have a message where the fee payer is Bob(Address A)/Phantom and I want to change it to Bob(Address A)/Solflare, this won't:

solana-web3.js/packages/signers/src/fee-payer-signer.ts

Lines 22 to 23 in 0936673

	if ('feePayerSigner' in transactionMessage)
	return transactionMessage as ITransactionMessageWithFeePayerSigner<TFeePayerAddress> & TTransactionMessage;

As discussed in #2874.