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

Are there points in the code the reviewer needs to double check?

Not really, since all changes made are very similar to those of !3230 (merged).

Why was this MR needed?

We (and possibly others) need this feature to launch specific jobs at a specific time (without using workarounds).

What are the relevant issue numbers?

Fixes #32955 (closed) - Run jobs only when they are run by a scheduled pipeline