GitHub Service sends status updates for pipelines

What

Adds new GithubService for sending status notifications to GitHub using pipeline hooks.

This allows a red/yellow/green status badge to appear on commits in GitHub, as well as providing a more detailed status on pull requests.

Why

We want a smoother experience when using projects for CI/CD only (https://gitlab.com/gitlab-org/gitlab-ee/issues/4839).

Having feedback in the GitHub pull request allows users to benefit from CI/CD updates without leaving the tool they are working from.

Screenshots

Acceptance criteria

• Changelog entry added, if necessary
• Documentation created/updated See: https://gitlab.com/gitlab-org/gitlab-ee/blob/jej/github-project-service-for-ci/doc/user/project/integrations/github.md
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)
• Feature assurance
• Documentation
• Added to features.yml

Related

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/3836