chore: Added cron job to check for upstream changes to vendored in code

[jamescrosswell]

Resolves #4995

• Create watch on upstream repositories that we've vendored in #4995

Notes for testing

All the github workflow does is run the script on a cron schedule, passing it some parameters. The script can be tested locally by running:

export GH_TOKEN="$(gh auth token)"
export GH_REPO="getsentry/sentry-dotnet"
./scripts/watch-upstream.sh dotnet/android tools/assembly-store-reader-mk2 src/Sentry.Android.AssemblyReader/

... that assumes you're logged into the github cli locally.

Important

The script will create one (and only one) issue for every upstream change it detects affecting the configured source tree. For testing, it created this issue:

• Upstream change detected: dotnet/android tools/assembly-store-reader-mk2 @ 740256f #5001
  ... so it's not going to do anything again until we see a new commit affecting that source tree.

#skip-changelog

[github-actions]

Semver Impact of This PR

⚪ None (no version bump detected)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.89%. Comparing base (ec307a7) to head (ae0aa16).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5000      +/-   ##
==========================================
- Coverage   73.90%   73.89%   -0.02%     
==========================================
  Files         497      497              
  Lines       17957    17957              
  Branches     3516     3516              
==========================================
- Hits        13271    13269       -2     
- Misses       3825     3826       +1     
- Partials      861      862       +1     

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