Add Scene name to `Unity` context

[bruno-garcia]

trafficstars

Some events like OOM and ANR are not very actionable due to lack of proper stack trace. At least knowing which Scene would help the user get an idea of where the problem happened.

This consists of the following steps:

• [x] Let the SDK report the active scene's name: https://github.com/getsentry/sentry-unity/pull/2084
• [ ] Prettify active_scene_name
• [ ] Automatically promote the active_scene_name to a tag to get it indexed and searchable