Feature: support events in GitLab CI `when` clause

Description

CI should support the notion of events, similar to Drone.

This would allow users to perform specific commands in reaction to specific events. The events should include all those that currently exist for webhooks.

An example use case would be performing a release upon merging any MR to master. (Note that this requires creating and pushing additional commits and tags during CI).

Links / references

Drone: Limit Steps by Event

Documentation blurb

when can also be used to limit jobs to only run in response to a specific event.

For example:

release_job:
  stage: release
  when: merge
  script:
  - make release