Add :waiting_for_callback CI status

What does this MR do and why?

Adds waiting_for_callback to the list of possible states for CI objects (eg. jobs/stages/pipelines).

This state represents jobs that are due to run, but instead of being picked by a runner will wait until the callback is received.

Note that there's currently no way for an object to enter this state, as there is no transition defined at the job level (the new transitions for pipeline and stage are to satisfy the test suite, but they are never actually called).

For more information, see the epic.

Screenshots or screen recordings

Pipeline view	Jobs list

How to set up and validate locally

1. Create a project, and add a minimal .gitlab-ci.yml such as:

   test:
     stage: test
     when: manual # So the job won't get picked by a runner
     script:
       - echo 'test'

2. In a Rails console, update the state of the job, stage and pipeline:

   job = Ci::Build.find <job-id>
   job.update! status: :waiting_for_callback
   job.ci_stage.update! status: :waiting_for_callback
   job.pipeline.update! status: :waiting_for_callback

3. View the pipeline and job pages to see the state text and icons.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #415736