Release generation from within `.gitlab-ci.yml`

Problem to solve

Now that we have a releases page (https://gitlab.com/gitlab-org/gitlab-ce/issues/41766), there should be a straightforward way to create a release from the .gitlab-ci.yml without making curls to the Releases API.

Target audience

Automation engineers who are working on releases and want a native way to add releases without using script: elements to curl an API

Further details

This is good polish for the releases feature in that it makes the integration feel better integrated into the product. Also, this provides better overall experience once features like https://gitlab.com/gitlab-org/gitlab-ce/issues/56023 are implemented.

Proposal

Introduce a new job syntax that can create a release automatically using POST /projects/:id/releases behind the scenes:

stages:
- build
- test
- release

build-osx:
  stage: build
  # ...
  artifacts:
    # ...

build-linux:
  stage: build
  # ...
  artifacts:
    # ...

release:
  stage: release
  only: tags
  script:
    - make changelog | tee release_changelog.txt
  release:
    name: My $CI_PROJECT_NAME Release 1.0-$CI_COMMIT_SHORT_SHA
    description: ./release_changelog.txt
    assets:
    - name: cool-app.zip
      url: http://my.awesome.download.site/1.0-$CI_COMMIT_SHORT_SHA.zip
    - name: cool-app.dmg
      path: out/cool-app.dmg
    - name: document-${APP_VERSION}.pdf
      path: out/doc/document.pdf

• release indicates that the job produces release asset(s). It is active only if git reference is tag
• release:name specifies the release name. If it's not created yet, then create a new entry.
• release:description specifies a file containing the description of the release (can point to changelog)
• release:assets adds assets to the release.
• release:assets:name specified the asset name. If the same name of an asset already existed, then overwrite url or path.
• release:assets:url specified the link of an asset (Implying asset type is Link).
• release:assets:path specified the binary path of an artifact (Implying asset type is Package). GitLab automatically transfer the result from the artifact to package.