Feature request: PSBT (missing) signature info

[andreasgriffin]

Describe the enhancement
for each input of a PSBT:

• contained and missing signatures (in multisig case). A possibility to identify the signers, for example via the fingerprint, or some other unique identifier.

Use case
When a psbt is loaded into a wallet, it is necessary to tell the user which signatures he acquired successfully, and which are still missing.