[WIP] feat(feedback): Add SentryEvent.get_feedback()

[limbonaut]

The before_send_feedback callback receives the prepared feedback event, but the fields the user submitted, the message, name, contact email, and associated event id, sit in that event's feedback context, which had no getter. A callback could discard the feedback or tag it, but could not read or scrub what was typed short of inspecting to_json(), and had no way to write anything back. This PR adds SentryEvent.get_feedback(), which returns the feedback as a live view of the event: reading it gives back what the user submitted, and writing to it modifies the event before it is sent. It returns null for events that carry no feedback, including the ones reaching before_send. iOS and macOS are the exception, since before_send_feedback is not implemented there yet.