[firebase_messaging] Support getting the instance ID in addition to the token.

Is your feature request related to a problem? Please describe.
Currently firebase_messaging only supports getting the instance ID token. Our use case requires the stable ID. This is not currently exposed in the flutter plugin.

Describe the solution you'd like
A method for retrieving the stable instance ID.

[talent-apps]

See also: #1924

@talent-apps FYI #2328

[Ehesp]

Dup of #2327

@Ehesp this is a dupe of itself?