fix!: Differentiate merge method of pull request and merge queue

[stanfordpeng]

BREAKING CHANGE: MergeMethod* consts have been split into: PullRequestMergeMethod* and MergeQueueMergeMethod*.

Fixes: #3558

This issue will cause GitHub API return error if user use type MergeMethod in MergeQueueRuleParameters.MergeMethod:

422 Invalid request. Invalid property /rules/0: data matches no possible input. See `documentation_url`. []

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.24%. Comparing base (6a7684f) to head (357a569).
Report is 5 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #3559 +/- ##
=======================================
Coverage 91.23% 91.24% =======================================
Files 183 183 Lines 16053 16063 +10 =======================================
+ Hits 14646 14656 +10 
Misses 1231 1231 Partials 176 176 

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[gmlewis]

Thank you, @stanfordpeng!
LGTM.

Awaiting second LGTM+Approval from any other contributor to this repo before merging.

@stevehipwell - might you be available for a code review? Thank you!

[stevehipwell]

LGTM

[gmlewis]

Thank you, @stevehipwell!
Merging.