Inject reserved pipeline policy stages

What does this MR do and why?

This MR adds support for reserved stages.

A policy can declare its jobs in the following stages:

• .pipeline-policy-pre - runs before .pre
• .pipeline-policy-post - runs after .post

These stages cannot be used by project CI configuration.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Stages cannot be used by project CI configuration:

Stages don't show up in the available stages list:

---

If used from a policy, they are injected into the pipeline:

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:pipeline_execution_policy_type)

2. Create a group
3. 1. In the group, create a Compliance project
4. In the Compliance project, create a file reserved-stages.yml:

   build job:
     stage: .pipeline-policy-pre
     script:
       - sleep 3 && echo "Policy building..."
   slow policy test job:
     stage: test
     script:
       - sleep 15 && echo "Slow test completed."
   policy job after build:
     stage: .pipeline-policy-post
     needs:
       - "build job"
     script:
       - echo "Should start right after 'build job' finishes."
   policy deploy job:
     stage: .pipeline-policy-post
     script:
       - echo "Deploying..."

5. In the group, create a new project SPP project.
6. In the project, create a file .gitlab/security-policies/policy.yml with the following content:

   ---
   pipeline_execution_policy:
     - name: Reserved stages policy
       description: ''
       enabled: true
       pipeline_config_strategy: inject_ci
       content:
         include:
           - project: <group-path>/compliance-project
             file: reserved-stages.yml
             ref: main

7. Create another project in the group: Test
8. In the project Test, go to Secure -> Policies, edit the policy project and select SPP project
9. In the project Test, create .gitlab-ci.yml:

   stages:
     - build
     - test
     - deploy
   
   build job:
     stage: build
     script:
       - echo "Compiling the code..."
       - echo "Compile complete."
   
   project test job:
     stage: test
     script:
       - echo "Running unit tests... This will take about 60 seconds."
       - echo "Code coverage is 90%"
   
   deploy job:
     stage: deploy
     environment: production
     script:
       - echo "Deploying application..."
       - echo "Application successfully deployed."

10. Go to Pipelines and run pipeline
11. Verify that stages with their jobs from the policy are injected into the pipeline in the expected order
12. Try to edit the project's .gitlab-ci.yml and try to assign a job to one of the restricted stages. Verify that this is not possible.

Related to #452384 (closed)