(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.
```

{
    {
        LOCK(cs_wallet);
        if (IsLocked())
            return false;
        CWalletDB walletdb(strWalletFile);
        
        unsigned int nTargetSize;
        if (kpSize > 0)
            nTargetSize = kpSize;
        else
            nTargetSize = max(GetArg("-keypool", DEFAULT_KEYPOOL_SIZE), (int64_t) 0);
        while (setKeyPool.size() < (nTargetSize + 1))
        {
            int64_t nEnd = 1;
            if (!setKeyPool.empty())
                nEnd = *(--setKeyPool.end()) + 1;
            if (!walletdb.WritePool(nEnd, CKeyPool(GenerateNewKey())))
                throw runtime_error("TopUpKeyPool(): writing generated key failed");
            setKeyPool.insert(nEnd);
            LogPrintf("keypool added key %d, size=%u\n", nEnd, setKeyPool.size());
        }
    }
    return true;
}
void CWallet::ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& keypool)