Add hooks:pre_get_sources_script to jobs/request

What does this MR do and why?

❗ This MR targets the branch of !103770 (merged). The target branch needs to be master before the merge. ❗

This MR adds the hooks:pre_get_sources_script to Runner's jobs/request.

Note: The changes are behind a feature flag ci_hooks_pre_get_sources_script (#381840 (closed)).

• Related to #356850 (closed)

New response

{
  "hooks": [
    {"name": "pre_get_sources_script", "script": ["line 1", "line 2"]}
  ]
}

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.	<-- This MR
4. Add documentation and CI schema	!104371 (merged)

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.