When testing a transaction for mempool acceptance, the fee must be calculated anyway. Thus, with minimal complexity overhead, the testmempoolaccept RPC could return the fee to the user.

#### Useful skills:

Basic understanding of the mempool, the Bitcoin Core RPC interface, and the functional test framework.

#### Want to work on this issue?

The purpose of the `good first issue` label is to highlight which issues are suitable for a new contributor without a deep understanding of the codebase.

You do not need to request permission to start working on this. You are encouraged to comment on the issue if you are planning to work on it. This will help other contributors monitor which issues are actively being addressed and is also an effective way to request assistance if and when you need it.

For guidance on contributing, please read [CONTRIBUTING.md](https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md) before opening your pull request.

{
            RPCHelpMan{"gettxoutproof",
                "\nReturns a hex-encoded proof that \"txid\" was included in a block.\n"
                "\nNOTE: By default this function only works sometimes. This is when there is an\n"
                "unspent output in the utxo for this transaction. To make it always work,\n"
                "you need to maintain a transaction index, using the -txindex command line option or\n"
                "specify the block in which the transaction is included manually (by blockhash).\n",
                {
                    {"txids", RPCArg::Type::ARR, RPCArg::Optional::NO, "The txids to filter",
                        {
                            {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::OMITTED, "A transaction hash"},
                        },
                        },
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::OMITTED_NAMED_ARG, "If specified, looks for txid in the block with this hash"},
                },
                RPCResult{
                    RPCResult::Type::STR, "data", "A string that is a serialized, hex-encoded data for the proof."
                },
                RPCExamples{""},
            }.Check(request);
    std::set<uint256> setTxids;
    uint256 oneTxid;
    UniValue txids = request.params[0].get_array();
    for (unsigned int idx = 0; idx < txids.size(); idx++) {
        const UniValue& txid = txids[idx];
        uint256 hash(ParseHashV(txid, "txid"));
        if (setTxids.count(hash)) {