Hide ITransaction and expose only ISpan as public API

[marandaneto]

This would change the signature of the startTransaction methods (Sentry/Hub/Scope), maybe ISentryClient too.
We'll need to update the docs and migration page.

cc @Tyrrrz @brustolin

[adinauer]

Once support for OpenTelemetry has landed and we look at spans differently then we can reconsider this.