add shared-change-checker CI step

[matt-codecov]

should automatically post/update a comment on PRs that change the hash of our shared dependency for reviewers to take a quick glance at

example comment (which was updated automatically as its PR switched to different hashes for shared):

[codecov-notifications]

Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.14%. Comparing base (4b7e59a) to head (6afdee2).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##               main       #797   +/-   ##
===========================================
  Coverage   96.14000   96.14000           
===========================================
  Files           812        812           
  Lines         18483      18483           
===========================================
  Hits          17771      17771           
  Misses          712        712           

Flag	Coverage Δ
unit	91.99% <ø> (ø)
unit-latest-uploader	91.99% <ø> (ø)

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