Add variable support for services

What does this MR do?

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 Go so there might be some issues that I'd be glad to fix with your help.

Why was this MR needed?

This is related to gitlab!72025 (merged), which aims to close gitlab#23671 (or at least to provide an initial solution).

What's the best way to test this MR?

1. Setup a new project
2. Register either a Docker or a Kubernetes executor
3. 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"