feat: add fleeting autoscale support closes #372

[MartinGodzina]

This PR introduces support for GitLab runners utilizing fleeting-based autoscaling. It enables autoscaling configurations for AWS, GCP, or Azure fleeting plugins.

New Feature
The following YAML input is now supported for configuring autoscaling:

gitlab_runner_runners:
  - name: "runner-name" 
    token: "runner-token" 
    executor: "docker-autoscaler"
    docker_image: "alpine"

    autoscaler:
      plugin: "azure"
      capacity_per_instance: 1
      max_use_count: 1
      max_instances: 10

      plugin_config:
        name: "scaleset-name" 
        subscription_id: "subs-id" 
        resource_group_name: "rg-name" 

      connector_config:
        username: "azureuser"
        use_static_credentials: true
        timeout: "10m"
        use_external_addr: true

      policies:
        - idle_count: 5
          idle_time: "20m0s"

closes #372

[riemers]

Thanks for the contribution