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.
return true;
    case TX_PUBKEYHASH: {
        CKeyID keyID = CKeyID(uint160(vSolutions[0]));
        if (!CreateSig(creator, sigdata, provider, sig, keyID, scriptPubKey, sigversion)) return false;
        ret.push_back(std::move(sig));
        CPubKey pubkey;
        GetPubKey(provider, sigdata, keyID, pubkey);
        ret.push_back(ToByteVector(pubkey));
        return true;
    }
    case TX_SCRIPTHASH:
        if (GetCScript(provider, sigdata, uint160(vSolutions[0]), scriptRet)) {
            ret.push_back(std::vector<unsigned char>(scriptRet.begin(), scriptRet.end()));
            return true;
        }
        return false;
    case TX_MULTISIG: {
        size_t required = vSolutions.front()[0];
        ret.push_back(valtype()); 
        for (size_t i = 1; i < vSolutions.size() - 1; ++i) {
            CPubKey pubkey = CPubKey(vSolutions[i]);
            if (ret.size() < required + 1 && CreateSig(creator, sigdata, provider, sig, pubkey.GetID(), scriptPubKey, sigversion)) {
                ret.push_back(std::move(sig));
            }
        }
        bool ok = ret.size() == required + 1;
        for (size_t i = 0; i + ret.size() < required + 1; ++i) {
            ret.push_back(valtype());
        }
        return ok;