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.
{
    SignatureData data;
    assert(tx.vin.size() > nIn);
    data.scriptSig = tx.vin[nIn].scriptSig;
    data.scriptWitness = tx.vin[nIn].scriptWitness;
    Stacks stack(data);
    
    MutableTransactionSignatureChecker tx_checker(&tx, nIn, txout.nValue);
    SignatureExtractorChecker extractor_checker(data, tx_checker);
    if (VerifyScript(data.scriptSig, txout.scriptPubKey, &data.scriptWitness, STANDARD_SCRIPT_VERIFY_FLAGS, extractor_checker)) {
        data.complete = true;
        return data;
    }
    
    std::vector<std::vector<unsigned char>> solutions;
    txnouttype script_type = Solver(txout.scriptPubKey, solutions);
    SigVersion sigversion = SigVersion::BASE;
    CScript next_script = txout.scriptPubKey;
    if (script_type == TX_SCRIPTHASH && !stack.script.empty() && !stack.script.back().empty()) {
        
        CScript redeem_script(stack.script.back().begin(), stack.script.back().end());
        data.redeem_script = redeem_script;
        next_script = std::move(redeem_script);
        
        script_type = Solver(next_script, solutions);
        stack.script.pop_back();
    }
    if (script_type == TX_WITNESS_V0_SCRIPTHASH && !stack.witness.empty() && !stack.witness.back().empty()) {