Add `services:variables` to CI schema

What does this MR do and why?

For #389498 (comment 1266799283)

This adds the services:variables definition to the CI schema and additional tests for the services keywords.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to the Pipeline Editor (CI/CD > Editor) and add the following to the .gitlab-ci.yml file.

   job:
     services:
       - name: bitnami/nginx
         alias: nginx
         variables:
           NGINX_HTTP_PORT_NUMBER: ${NGINX_HTTP_PORT_NUMBER}
     script: echo NGINX_HTTP_PORT_NUMBER

2. When hovering over the services:variables keyword, the annotation should show up.

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.