Add support for merge queue ruleset JSON unmarshaling

[Matthew-Reidy]

Fixes: #3098.

Added support for respository ruleset merge_queue in JSON unmarshalling code with accompanying tests

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[gmlewis]

Thank you, @Matthew-Reidy !
Just a couple minor tweaks, please, then we should be ready for a second LGTM+Approval from any other contributor to this repo before merging.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.88%. Comparing base (2b8c7fa) to head (db98743).
Report is 41 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #3131 +/- ##
==========================================
- Coverage 97.72% 92.88% -4.84% 
==========================================
Files 153 170 +17 Lines 13390 11426 -1964 ==========================================
- Hits 13085 10613 -2472 - Misses 215 723 +508 
Partials 90 90 

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[Matthew-Reidy]

Sure thing. I'll fix those items shortly

[gmlewis]

Thank you, @Matthew-Reidy !
LGTM.

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

[liaodaniel]

Just a minor comment to update but looks good otherwise

[Matthew-Reidy]

changes to that comment have been made

[liaodaniel]

LGTM

[gmlewis]

Thank you, @liaodaniel !
Merging.