(from discussion on IRC. Currently a global `settxfee` has to be changed to override the fee rate, it would make sense to be able to specify it on a per-transaction basis. This would continue #7518)

```
-*- jonasschnelli thinks we should probably add a optional feerate parameter to fundrawtransaction
<wumpus> jonasschnelli: there isn't one yet?
-*- jonasschnelli looking..
<wumpus> I'm surprised, would be so much better than changing a global
<jonasschnelli> no.. i don't think so. Maybe there is a PR.
<sipa> there is a pr
<jonasschnelli> Hard to keep track of everything. :)
<wumpus> no, the PR does it wrong
```

Regarding implementation:

```
<jonasschnelli> The ugly thing is, that CreateTransaction uses the static DEFAULT_TARGET
<wumpus> passing in an optional feerate would be the obvious thing to do
<jonasschnelli> So, this would require some refactoring.
<sipa> just add a confirmation target to coincontrol
<jonasschnelli> Yes. This sounds good.
<wumpus> right, just slap it into coincontrol
<sipa> wasn't there a PR that added a bunch of options to fundrawtransaction?
<sipa> including that
<jonasschnelli> Not the feerate.
<jonasschnelli> But the rest is merged.
```

{
    return std::max(minTxFee.GetFee(nTxBytes), ::minRelayTxFee.GetFee(nTxBytes));
}
CAmount CWallet::GetMinimumFee(unsigned int nTxBytes, unsigned int nConfirmTarget, const CTxMemPool& pool)
{
    
    CAmount nFeeNeeded = payTxFee.GetFee(nTxBytes);
    
    if (nFeeNeeded == 0) {
        int estimateFoundTarget = nConfirmTarget;
        nFeeNeeded = pool.estimateSmartFee(nConfirmTarget, &estimateFoundTarget).GetFee(nTxBytes);
        
        if (nFeeNeeded == 0)
            nFeeNeeded = fallbackFee.GetFee(nTxBytes);
    }
    
    nFeeNeeded = std::max(nFeeNeeded, GetRequiredFee(nTxBytes));
    
    if (nFeeNeeded > maxTxFee)
        nFeeNeeded = maxTxFee;
    return nFeeNeeded;
}
DBErrors CWallet::LoadWallet(bool& fFirstRunRet)