feat(github): Add checkrun support

[davhdavh]

• Cleans up the github code so that each new template-supported prop only takes a single line (instead of 10+)
• Add support for CheckRuns
• Add a bit of documentation for all of the supported APIs

[davhdavh]

That linter is really annoying, could it not just give ALL of the errors in 1 run, instead of taking 8...

anyway, how to solve the last one?

  Error: composites: github.com/google/go-github/v41/github.Timestamp struct literal uses unkeyed fields (govet)

[codecov[bot]]

Codecov Report

Attention: 74 lines in your changes are missing coverage. Please review.

Comparison is base (a8d185e) 55.13% compared to head (ae1870f) 54.74%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #223 +/- ## ========================================== - Coverage 55.13% 54.74% -0.40% ========================================== Files 35 35 Lines 3337 3352 +15 ========================================== - Hits 1840 1835 -5 - Misses 1230 1263 +33 + Partials 267 254 -13 ``` | [Files](https://app.codecov.io/gh/argoproj/notifications-engine/pull/223?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argoproj) | Coverage Δ | | |---|---|---| | [pkg/services/github.go](https://app.codecov.io/gh/argoproj/notifications-engine/pull/223?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argoproj#diff-cGtnL3NlcnZpY2VzL2dpdGh1Yi5nbw==) | `32.05% <39.34%> (-3.61%)` | :arrow_down: |

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[davhdavh]

There is no existing tests for the api calls, so I have no idea how to make test for the new api call.