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.
{
    
    const auto it = sigdata.signatures.find(address);
    if (it != sigdata.signatures.end()) {
        pubkey = it->second.first;
        return true;
    }
    
    const auto& pk_it = sigdata.misc_pubkeys.find(address);
    if (pk_it != sigdata.misc_pubkeys.end()) {
        pubkey = pk_it->second.first;
        return true;
    }
    
    if (provider.GetPubKey(address, pubkey)) {
        KeyOriginInfo info;
        if (provider.GetKeyOrigin(address, info)) {
            sigdata.misc_pubkeys.emplace(address, std::make_pair(pubkey, std::move(info)));
        }
        return true;
    }
    return false;
}
static bool CreateSig(const BaseSignatureCreator& creator, SignatureData& sigdata, const SigningProvider& provider, std::vector<unsigned char>& sig_out, const CKeyID& keyid, const CScript& scriptcode, SigVersion sigversion)