Add support for custom user agent strings

[cmelchior]

The latest Sync/ROS release added support for defining a custom user agent string that is sent when creating a session. This can help debugging and identifying problematic clients.

The user agent string consists of 3 parts:

1. A part defined by Sync
2. A part defined by us (Realm version / Device / OS version)
3. A part defined by the user

We need to implement 2) and expose 3) to end users.

Suggested API is:

Realm.init(Context context, String userAgent)

As the UserAgent can only be defined up to the point we the C++ SyncManager this will fit both that and the semantics that the user agent cannot be changed once the Client is started.

Internal ref: https://github.com/realm/realm-sync/issues/2455#issuecomment-432447520