Chore: Refactor HubExtensions and SentrySdk

[jamescrosswell]

Summary

See conversation thread here.

The HubAdapter passes it's work to SentrySdk.CurrentHub (a static instance of Hub). HubExtensions and SentrySDK both employ switch expression in multiple locations to determine whether to call a real internal implementation of IHubEx or the nearest equivalent public implementation of something from IHub (implicitly on DisabledHub, since it's the only hub that implements IHub but not IHubEx). That could probably be achieved more simply by having DisabledHub implemented IHubEx and then do the same (return default or whatever else the extension methods are doing).

[bitsandfoxes]

With getsentry/rfcs#122 on the horizon we can close this as it won't likely happen.