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 (redeem_script.empty() && !sigdata.redeem_script.empty()) {
        redeem_script = sigdata.redeem_script;
    }
    if (witness_script.empty() && !sigdata.witness_script.empty()) {
        witness_script = sigdata.witness_script;
    }
    for (const auto& entry : sigdata.misc_pubkeys) {
        hd_keypaths.emplace(entry.second);
    }
}
bool PSBTOutput::IsNull() const
{
    return redeem_script.empty() && witness_script.empty() && hd_keypaths.empty() && unknown.empty();
}
void PSBTOutput::Merge(const PSBTOutput& output)
{
    hd_keypaths.insert(output.hd_keypaths.begin(), output.hd_keypaths.end());
    unknown.insert(output.unknown.begin(), output.unknown.end());
    if (redeem_script.empty() && !output.redeem_script.empty()) redeem_script = output.redeem_script;
    if (witness_script.empty() && !output.witness_script.empty()) witness_script = output.witness_script;
}
bool HidingSigningProvider::GetCScript(const CScriptID& scriptid, CScript& script) const
{
    return m_provider->GetCScript(scriptid, script);
}
bool HidingSigningProvider::GetPubKey(const CKeyID& keyid, CPubKey& pubkey) const