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.
bool CheckSig(const std::vector<unsigned char>& scriptSig, const std::vector<unsigned char>& vchPubKey, const CScript& scriptCode, SigVersion sigversion) const override;
};
bool SignatureExtractorChecker::CheckSig(const std::vector<unsigned char>& scriptSig, const std::vector<unsigned char>& vchPubKey, const CScript& scriptCode, SigVersion sigversion) const
{
    if (checker.CheckSig(scriptSig, vchPubKey, scriptCode, sigversion)) {
        CPubKey pubkey(vchPubKey);
        sigdata.signatures.emplace(pubkey.GetID(), SigPair(pubkey, scriptSig));
        return true;
    }
    return false;
}
namespace
{
struct Stacks
{
    std::vector<valtype> script;
    std::vector<valtype> witness;
    Stacks() = delete;
    Stacks(const Stacks&) = delete;
    explicit Stacks(const SignatureData& data) : witness(data.scriptWitness.stack) {
        EvalScript(script, data.scriptSig, SCRIPT_VERIFY_STRICTENC, BaseSignatureChecker(), SigVersion::BASE);
    }
};
}
SignatureData DataFromTransaction(const CMutableTransaction& tx, unsigned int nIn, const CTxOut& txout)