Shared CI code between GitHub and GitLab

[matejchalk]

The code-pushup/github-action code has grown in complexity since monorepo mode was added. Since we want to have very similar functionality for GitLab also (see https://github.com/code-pushup/github-action/issues/58), it would be much more maintainable to make the core logic CI-agnostic and extract it to a shared NPM package, so that both the GitHub Action and the GitLab template can import it and build on top of it.

The proposal is to create a dedicated @code-pushup/ci package for this purpose, which would be distributed from code-pushup/cli repo.

Acceptance criteria

• [x] new @code-pushup/ci package is created in CLI monorepo
• [ ] create abstraction layer from source code in code-pushup/github-action which accepts configuration for CI provider
• [ ] publish @code-pushup/ci package and import it in code-pushup/github-action
  • [x] prerequisite: code-pushup/github-action#69
• [ ] test code-pushup/github-action still works and publish new version to marketplace