Add resource groups API endpoint to list upcoming jobs

What does this MR do and why?

This merge request adds a new endpoint in the Resource Groups API allowing users to list all upcoming jobs for a specific resource group.

Another merge request will follow with the documentation.

Resolves #342016 (closed).

How to set up and validate locally

• Start up your GDK if it is not already: gdk start.
• Create a new project, and create an environment: production in the same project from: Deployments > Environments.
• Add the following .gitlab-ci.yml configuration to the project:

image: alpine:latest

stages:
  - test
  - deploy

say_test:
  stage: test
  script: echo 'test!'

deploy_production:
  stage: deploy
  script: echo 'deploying..' && sleep 45
  resource_group: production
  environment: production

• Go to CI/CD > Pipelines, and run two or three pipelines simultaneously.
• In your terminal, send a request to the new endpoint like below, while keeping in mind to:
  • Use your preferred method of API authentication and update YOUR_TOKEN with your own.
  • Make sure to update project_id with the corresponding project id.

curl -X GET -H "Content-Type: application/json" -H "Authorization: Bearer YOUR_TOKEN" "http://localhost:3000/api/v4/projects/:project_id/resource_groups/production/upcoming_jobs"

4. Validate that the response includes the jobs still showing as pending in CI/CD > Pipelines.

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.