Expose short token in REST and GraphQL

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What does this MR do and why?

This MR adds the trigger_short_token field to the REST and GraphQL APIs as requested in #384360 (closed). This is needed to be able to check if a pipeline was triggered via a trigger token and to check which trigger token was used.

This MR also addds the field to the docs.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create a trigger token by navigating to Settings -> CI/CD -> Pipeline trigger tokens
2. Create a pipeline by sending a POST request to https://<instance url>/api/v4/projects/<project_id>/ref/<ref_name>/trigger/pipeline?token=<token>
3. Verify the job was created and the short token is visible in the job details
4. Send a GET request to /api/v4/projects/:id/jobs
5. Send the following GraphQL query:

query showShortToken {
  project(fullPath: "<project path>") {
    jobs {
      nodes {
        triggerShortToken
      }
    }
  }
}

6. Verify that trigger_short_token has a value