feat: add ID getter methods

[carsonoid]

Without this, it is not possible to know which app or installation a given transport is for without tracking it separately.

These are getter functions that return copies so they are backwards compatible and cannot cause any issues or break a transport.

[bradleyfalzon]

Thanks, this use case makes sense to me, as I can see someone may want to log the particular appId or similar when there's an error. Having a getter method makes sense instead of making it public due to the complexity involved in making that variable mutable. Also, thanks for the test.