Allow `set_user` to update current user instead of replacing it

[thomaseizinger]

In our application, not all user-identifying pieces of information are known at the same time. We would like to use set_user to add as much information as we can to the user context.

At present, sentry only allows the whole user to be set at once, meaning we have to track that information separately and update the user context every time we learn new information.

The simplest way to solve this would be if we could expose a user getter. That would allow the application to see, what the current user is and update it with the existing fields and add new ones to it. Alternatively, a user_mut could be exposed as well. If that would return &mut Option<User> then one could use the get_or_insert family of APIs to set / update the current user.