Allow an empty array of repo ids as a request parameter

[Matthew-Reidy]

Fixes: #3106.

Add extra endpoint function that allows client to pass an empty array of repository ids to the installations/<install_id>/access_tokens endpoint as a parameter. Response lists all repositories in an installation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.89%. Comparing base (2b8c7fa) to head (6f4c7c1).
Report is 44 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #3155 +/- ##
==========================================
- Coverage 97.72% 92.89% -4.84% 
==========================================
Files 153 170 +17 Lines 13390 11436 -1954 ==========================================
- Hits 13085 10623 -2462 - Misses 215 723 +508 
Partials 90 90 

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[gmlewis]

Thank you, @Matthew-Reidy !
One tiny nit so we have complete sentences in our godocs, otherwise LGTM.

[gmlewis]

Thank you, @Matthew-Reidy !
LGTM.

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

[gmlewis]

Thank you, @be0x74a !
Merging.