feat: implement CI pipeline status reporting when evaluating MR

[jippi]

Breaking change:

--commit / $CI_COMMIT_SHA variable

scm-engine evaluate now requires --commit (or env CI_COMMIT_SHA) to work.

The CI_COMMIT_SHA variable is provided in GitLab CI pipelines automatically; so hopefully shouldn't break anything.

--update-pipeline / $SCM_ENGINE_UPDATE_PIPELINE (default: true)

Projects without CI pipelines might fail due to lack of CI pipeline API; other projects should now see an external CI pipeline status from scm-engine showing success/failure + relevant error message when scm-engine is processing an MR

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 77 lines in your changes are missing coverage. Please review.

Project coverage is 0.00%. Comparing base (c35eee6) to head (97eb963).

Files	Patch %	Lines
pkg/scm/gitlab/client.go	0.00%	23 Missing ⚠️
pkg/state/context.go	0.00%	16 Missing ⚠️
cmd/cmd_server.go	0.00%	12 Missing ⚠️
cmd/cmd_evaluate.go	0.00%	11 Missing ⚠️
pkg/scm/gitlab/client_merge_request.go	0.00%	9 Missing ⚠️
cmd/shared.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #17   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files         30      30           
  Lines       1271    1319   +48     
=====================================
- Misses      1271    1319   +48     

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