Use case: **decouple keys from the wallet** (via importing scripts and fundrawtransaction with watch-only).

**Problem:** multisig inputs are flagged as non-spendable even if importing both the multisig P2SH `scriptPubKey` and the `redeemScript` (via importmulti).

The current `isSolveable()` logic calls `GetPubKey()` and only flags them as solvable if the pubkeys are **in the wallet**.

**Expected behaviour:**
* multisig Inputs should be flagged as solvable (and therefore usable via fundrawtransaction) when importing the P2(W)SH `scriptPubKey` and the `redeemScripts`

Current workaround:
* Call importpubkey for all pubkeys used in the multisig (redeemScript). importpubkey derives all coming scripts (including a raw P2PK script). The P2PK scripts make the wallet store the pubkey and flag the multisig script solvable.
std::vector<valtype>& ret, txnouttype& whichTypeRet, SigVersion sigversion, SignatureData& sigdata)
{
    CScript scriptRet;
    uint160 h160;
    ret.clear();
    std::vector<unsigned char> sig;
    std::vector<valtype> vSolutions;
    whichTypeRet = Solver(scriptPubKey, vSolutions);
    switch (whichTypeRet)
    {
    case TX_NONSTANDARD:
    case TX_NULL_DATA:
    case TX_WITNESS_UNKNOWN:
        return false;
    case TX_PUBKEY:
        if (!CreateSig(creator, sigdata, provider, sig, CPubKey(vSolutions[0]).GetID(), scriptPubKey, sigversion)) return false;
        ret.push_back(std::move(sig));