Add documentation and CI schema for hooks:pre_get_sources_script

What does this MR do and why?

This MR adds documentation and CI schema for hooks:pre_get_sources_script.

default:
  hooks:
    pre_get_sources_script:
      - echo 'hello default pre_get_sources'

job1:
  hooks:
    pre_get_sources_script:
      - echo 'hello job1 pre_get_sources'
  script: echo 'hello job1 script'

job2:
  script: echo 'hello job2 script'

• Related to #356850 (closed)

Implementation table

Runner counterpart: gitlab-runner!3713 (merged)

Step	Status
1. Add the basic job syntax.	!102332 (merged)
2. Add default:script_hooks.	!103770 (merged)
3. Add to JobRequest::Response for Runner.	!104254 (merged)
4. Add documentation and CI schema	<-- This MR

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.