draft: Hook for package installation trigger

[bitsandfoxes]

trafficstars

Supersedes https://github.com/getsentry/sentry-unity/pull/213

Goal: Having a package installation trigger for creating link.xml or prompting an installation window.

https://docs.unity3d.com/Manual/upm-api.html won't work for now because the events are not available in Unity 2019.4

#skip-changelog