feat: Add MergeAsync and GetMergeAsyncResult support

[ymh3090]

Adds support for the asynchronous pull request merge endpoints, needed for merging stacked pull requests (the existing Merge method doesn't support stacks).

• PullRequestsService.MergeAsync — PUT /repos/{owner}/{repo}/pulls/{pull_number}/merge-async
• PullRequestsService.GetMergeAsyncResult — GET /repos/{owner}/{repo}/pulls/{pull_number}/merge-async/{uuid}

Docs:

• https://docs.github.com/en/rest/pulls/pulls?apiVersion=2022-11-28#merge-a-pull-request-asynchronously
• https://docs.github.com/en/rest/pulls/pulls?apiVersion=2022-11-28#get-the-result-of-an-asynchronous-merge

Closes #4485

[google-cla]

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[ymh3090]

recheck

[ymh3090]

recheck

[ymh3090]

hi, @gmlewis !
I've signed the Google CLA, but the cla/google check is still failing after a recheck. Could you take a look?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.53%. Comparing base (34cec4f) to head (1a2d3d1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4491   +/-   ##
=======================================
  Coverage   98.53%   98.53%           
=======================================
  Files         195      195           
  Lines       17691    17709   +18     
=======================================
+ Hits        17431    17449   +18     
  Misses        260      260           

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[gmlewis]

Thank you, @ymh3090!
LGTM.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @stevehipwell - @alexandear - @Not-Dhananjay-Mishra

[alexandear]

Please improve naming for consistency.