Resolve "support all options from `docker-compose.yml` in gitlab runner services"

What does this MR do and why?

Currently, the only way to pass environment variables to a Docker/Kubernetes service during CI is to provide them inside the job's variables. This MR adds the possibility to declare an additional set of variables for the services, making them closer to the docker-compose spec.

Please note that it's my very first time using Ruby so there might be some issues that I'd be glad to fix with your help.

How to set up and validate locally

1. Setup a new project

2. Create a new .gitlab-ci.yml file with the following contents

   image: busybox:latest
   
   variables:
      GIT_STRATEGY: none
   
   stages:
      - build
   
   Build With A Service And Some Variables:
      stage: build
      image: curlimages/curl
      services:
      - name: ealen/echo-server:latest
        alias: echo-server
        variables:
          PORT: 5000
          ENABLE__ENVIRONMENT: 'true'
          ENV_VAR_FOR_SERVICE: "This is a variable passed to the service"
      script:
        - curl echo-server:5000/?echo_env_body=ENV_VAR_FOR_SERVICE
   
   Build With A Service And No Variables:
      stage: build
      image: curlimages/curl
      services:
      - name: ealen/echo-server:latest
        alias: echo-server
      script:
        - curl echo-server:80/
   
   Build Without A Service:
      stage: build
      image: busybox:latest
      script:
        - echo "Build Without A Service"

3. The CI file is correctly validated by the linter

4. It can also be tested by building and using a runner with the code from gitlab-runner!3158 (closed)

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.

Related to #23671