feat: Package update notification

[bitsandfoxes]

Works from 2020.3 onwards.
With something like this, we can detect updates and also from which version. So we can notify about things like experimental features no longer being experimental on the wizard.

Package changed from io.sentry.unity.dev-0.0.1 to io.sentry.unity.dev-0.0.2

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

- Package update notification ([#964](https://github.com/getsentry/sentry-unity/pull/964))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

Generated by 🚫 dangerJS against 3776d7a

[getsantry]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you remove the label Waiting for: Community, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀