feat(github): Support GitHub transient environment deployment type

[ricky-sb]

Many people use ArgoCD for preview deployments. GitHub announced transient environments a while back.

We've added a new transient_environment field to allow you to communicate that an environment is specific to a deployment and will no longer exist at some point in the future (e.g. a temporary testing environment like a Heroku Review App).

This PR adds support for that field.

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (7b19d2c) 55.16% compared to head (2eab075) 55.08%.

❗ Current head 2eab075 differs from pull request most recent head cf764e3. Consider uploading reports for the commit cf764e3 to get more accurate results

Files	Patch %	Lines
pkg/services/github.go	18.18%	7 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #260      +/-   ##
==========================================
- Coverage   55.16%   55.08%   -0.09%     
==========================================
  Files          35       35              
  Lines        3350     3344       -6     
==========================================
- Hits         1848     1842       -6     
  Misses       1233     1233              
  Partials      269      269              

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[ricky-sb]

Linter is unhappy but once we merge in #257 we should be good!

[pasha-codefresh]

LGTM, thank you