Move public types to a more public place

[sentrivana]

Problem Statement

See #4122

This is mostly about Event and Hint, but other types could be useful as well.

Currently, they are not easy to find -- IDEs have a hard time too. This makes writing custom callbacks like before_send harder than it should be.

Solution Brainstorm

We should consider moving them to types directly (instead of just re-exporting them there), or possibly even on the top level so that folks can just do from sentry_sdk import init, Event, Hint.

A small improvement for potel-base maybe?

[MarcoGlauser]

In general, please consider all types that are used for public apis also public.
One such example is MonitorConfig, which is required when manually instrumenting cron job monitoring. I understand you don't want people using internal stuff, but currently it's difficult to tell them apart because even supposedly public types are in the private namespace.