chore(pr-comments): generalization for triggering merged PR comments workflow

[cathteng]

If we want to be able to have merged PR comments for SCMs other than GitHub, we'll need to generalize some of the flows. In this PR I generalize the flow to trigger the merged PR comments workflow.

1. Check if the repo for the suspect commit has a provider that supports merged PR comments
2. Move the custom logic needed to fetch the merge commit sha from the commit sha into the GitHub client, this can be repeated for other SCMs. We use this to find the PR associated with the merge commit sha in our db.

In a follow up PR, I will generalize the workflow itself.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.16%. Comparing base (028e402) to head (43f79bd). Report is 148 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #71039      +/-   ##
==========================================
+ Coverage   78.13%   78.16%   +0.03%     
==========================================
  Files        6670     6672       +2     
  Lines      298465   298750     +285     
  Branches    51354    51427      +73     
==========================================
+ Hits       233212   233530     +318     
+ Misses      58993    58955      -38     
- Partials     6260     6265       +5     

Files	Coverage Δ
src/sentry/integrations/github/client.py	83.42% <100.00%> (+0.43%)	:arrow_up:
src/sentry/tasks/commit_context.py	98.38% <100.00%> (+2.32%)	:arrow_up:

... and 107 files with indirect coverage changes

[getsantry[bot]]

This pull request 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 add the label WIP, I will leave it alone unless WIP is removed ... forever!

---

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

[getsantry[bot]]

This pull request 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 add the label WIP, I will leave it alone unless WIP is removed ... forever!

---

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