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.
{
    if (sigdata.complete) return true;
    std::vector<valtype> result;
    txnouttype whichType;
    bool solved = SignStep(provider, creator, fromPubKey, result, whichType, SigVersion::BASE, sigdata);
    bool P2SH = false;
    CScript subscript;
    sigdata.scriptWitness.stack.clear();
    if (solved && whichType == TX_SCRIPTHASH)
    {
        
        
        
        subscript = CScript(result[0].begin(), result[0].end());
        sigdata.redeem_script = subscript;
        solved = solved && SignStep(provider, creator, subscript, result, whichType, SigVersion::BASE, sigdata) && whichType != TX_SCRIPTHASH;
        P2SH = true;
    }
    if (solved && whichType == TX_WITNESS_V0_KEYHASH)
    {