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.

{RPCResult::Type::ARR, "inputs", "",
                    {
                        {RPCResult::Type::OBJ, "", "",
                        {
                            {RPCResult::Type::BOOL, "has_utxo", "Whether a UTXO is provided"},
                            {RPCResult::Type::BOOL, "is_final", "Whether the input is finalized"},
                            {RPCResult::Type::OBJ, "missing",  true, "Things that are missing that are required to complete this input",
                            {
                                {RPCResult::Type::ARR, "pubkeys",  true, "",
                                {
                                    {RPCResult::Type::STR_HEX, "keyid", "Public key ID, hash160 of the public key, of a public key whose BIP 32 derivation path is missing"},
                                }},
                                {RPCResult::Type::ARR, "signatures",  true, "",
                                {
                                    {RPCResult::Type::STR_HEX, "keyid", "Public key ID, hash160 of the public key, of a public key whose signature is missing"},
                                }},
                                {RPCResult::Type::STR_HEX, "redeemscript",  true, "Hash160 of the redeemScript that is missing"},
                                {RPCResult::Type::STR_HEX, "witnessscript",  true, "SHA256 of the witnessScript that is missing"},
                            }},
                            {RPCResult::Type::STR, "next",  true, "Role of the next person that this input needs to go to"},
                        }},
                    }},
                    {RPCResult::Type::NUM, "estimated_vsize",  true, "Estimated vsize of the final signed transaction"},
                    {RPCResult::Type::STR_AMOUNT, "estimated_feerate",  true, "Estimated feerate of the final signed transaction in " + CURRENCY_UNIT + "/kB. Shown only if all UTXO slots in the PSBT have been filled"},
                    {RPCResult::Type::STR_AMOUNT, "fee",  true, "The transaction fee paid. Shown only if all UTXO slots in the PSBT have been filled"},
                    {RPCResult::Type::STR, "next", "Role of the next person that this psbt needs to go to"},
                    {RPCResult::Type::STR, "error",  true, "Error message (if there is one)"},
                }
            },
            RPCExamples {