Draft: External jobs PoC

What does this MR do and why?

Not to be merged. Spike for #427389 (closed).

Screenshots or screen recordings

How to set up and validate locally

1. Create a personal access token
2. Create a new project, and add the following .gitlab-ci.yml:

   stages:
     - build
     - sync
     - report
   
   build:
     stage: build
     script:
       - echo 'build'
   
   sync:
     stage: sync
     external: true
     environment: production
     timeout: 10m
   
   report:
     stage: report
     script:
       - echo 'report'

3. Make a note of the ID of your project, and the SHA for the commit you just created
4. View the created pipeline, and wait until the build job has finished (should be almost instant)
5. Observe the sync job "starts", but is not actually running via runner
6. From a terminal, execute the following, replacing TOKEN, PROJECT_ID, BRANCH and SHA with the relevant values:

   curl --request POST \
        --header "PRIVATE-TOKEN: <TOKEN>" \
        "https://gdk.test:3443/api/v4/projects/<PROJECT_ID>/pipeline_callback?ref=<BRANCH>&sha=<SHA>&job_name=sync&job_status=success"

7. Watch the pipeline page, and observe the sync job completes successfully, the report job starts and completes, and the pipeline succeeds
8. [Optional] Start another pipeline, and execute the same curl command, this time with job_status=failed. The pipeline will fail as expected
9. [Optional] Adjust the timeout in the external job definition to 1m. The pipeline will fail by itself when the timeout is reached

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.