chore: typed returns for MultiPassBox.get_identity(...) in typescript

[ashneverdawn]

When MultiPassBox.get_identity(...) is exported to typescript, the return type is Promise<any>
We want any to be typed. (In rust it's a Vec<Identity>).

Furthermore, consider splitting .get_identity(...) into separate functions for it's variants (DID, DIDList, Username) so that DID and Username variants can return just an Identity instead of a Vec<Identity> with a single element.

[dariusc93]

Given that MultiPass::get_identity is now returns a stream, we could probably convert the exported function to return an asynciter for the stream side (by default) and have a separate function for a single identity. See Satellite-im/Warp#554