Add hooks:pre_get_sources_script syntax for CI config

What does this MR do and why?

This MR adds a new hooks:pre_get_sources_script syntax to the CI config.

We have pre_clone_script in the Runner config. Here, we are providing the same config in the CI YAML.

pre_clone_script from the documentation;

Commands to be executed on the runner before cloning the Git repository. Use it to adjust the Git client configuration first, for example.

Note: The changes are behind a feature flag ci_hooks_pre_get_sources_script (#381840 (closed)). And I used this FF only to prevent returning hooks from the job entry for now.

• Related to #356850 (closed)

New syntax

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

Implementation table

Runner counterpart: gitlab-runner!3713 (merged)

Step	Status
1. Add the basic job syntax.	<-- This MR
2. Add default:script_hooks.	!103770 (merged)
3. Add to JobRequest::Response for Runner.	!104254 (merged)
4. Add documentation and CI schema	!104371 (merged)

MR acceptance checklist

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