Add all pipeline sources as special keywords to 'only' and 'except'

What does this MR do?

This MR adds the possibility to add the following new special keywords to the only and except list in a .gitlab-ci.yml file:

• api
• external
• pushes
• schedules
• web

Originally contributed by @krakowski in gitlab-org/gitlab-ce!11844

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug

What are the relevant issue numbers?

Closes #32955 (closed)