Add option to import "individual" multisignature address keys

[Coding-Enthusiast]

Electrum already supports MultiSig wallets and users can create such wallets as long as they use seed words and/or master keys but there is no option to import individual keys and redeem-scrip for a single MultiSig address and be able to spend from it.
Please add this feature.

[gits7r]

Indeed I see the usefulness of such a feature. The problem is, it's not possible to have it still available after wallet restore since there's nothing deterministic about it.

It will be like an imported wallet inside a different wallet (standard, 2FA, etc). So best option would be to add this in a separate tab that will warn users accordingly.

Or we could only improve our sweep feature from multisig utxo's, where the utxo must be spendable (not time locked, other conditions, etc.) and user will just provide the required private keys to unlock and the redeem script, spending all the inputs available to an output address from the "default" wallet that is used at this particular instance.

[SomberNight]

related: #5694