Make ITransport and SentryOptions.Transport public

[mattjohnsonpint]

In #1560, we created the HttpTransportBase class, containing the "guts" of the HttpTransport assembled such that others could extend it to create their own transports without loss of features that are wired at the transport. However, we didn't create an easy way to actually attach a custom transport at that time, because we were primarily focused on getsentry/sentry-unity#657, which was already building it's own implementation of IBackgroundWorker where it could connect its own custom transport.

For others, the ideal place to attach a custom transport is on SentryOptions.Transport, as we do in unit tests. This PR makes that property, and the corresponding ITransport type public so that can happen. (It also removes IFlushableTransport just for cleanup. It wasn't adding anything.)

After this is released, the recommendation for custom transports is to implement similarly to the built-in HttpTransport.

public class SomeCustomTransport : HttpTransportBase, ITransport

One could also just implement ITransport without implementing HttpTransportBase, but then they would be missing out on any functionality wired up in the transport such as processing event handlers, client reports, etc.

[bruno-garcia]

Thanks Matt