(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.
```

/**
     * populate vCoins with vector of available COutputs.
     */
    void AvailableCoins(std::vector<COutput>& vCoins, bool fOnlyConfirmed=true, const CCoinControl *coinControl = NULL, bool fIncludeZeroValue=false) const;
    /**
     * Shuffle and select coins until nTargetValue is reached while avoiding
     * small change; This method is stochastic for some inputs and upon
     * completion the coin set and corresponding actual target value is
     * assembled
     */
    bool SelectCoinsMinConf(const CAmount& nTargetValue, int nConfMine, int nConfTheirs, std::vector<COutput> vCoins, std::set<std::pair<const CWalletTx*,unsigned int> >& setCoinsRet, CAmount& nValueRet) const;
    bool IsSpent(const uint256& hash, unsigned int n) const;
    bool IsLockedCoin(uint256 hash, unsigned int n) const;
    void LockCoin(const COutPoint& output);
    void UnlockCoin(const COutPoint& output);
    void UnlockAllCoins();
    void ListLockedCoins(std::vector<COutPoint>& vOutpts);
    /**
     * keystore implementation
     * Generate a new key
     */
    CPubKey GenerateNewKey();
    
    bool AddKeyPubKey(const CKey& key, const CPubKey &pubkey);
    
    bool LoadKey(const CKey& key, const CPubKey &pubkey) { return CCryptoKeyStore::AddKeyPubKey(key, pubkey); }