Allow service alias from config in Kubernetes executor

Overview

A user can define an alias when defined services inside of the .gitlab-ci.yml, but not inside of the config.toml the user can't. Similar to #4114 (closed) the user should be able to define a host alias to the service for the Kubernetes executor once #2229 (closed) has been implemented.

Proposal

Update the config struct to accept alias for services and define it when creating services.

concurrent = 1
check_interval = 30
  [[runners]]
    name = "myRunner"
    url = "gitlab.example.com"
    executor = "kubernetes"
    [runners.kubernetes]
      helper_image = "gitlab-registy.example.com/helper:latest"
      [[runners.kubernetes.services]]
        name = "postgres:12-alpine"
        alias = "somedb"
      [[runners.kubernetes.services]]
        name = "percona:latest"
        alias = "otherdb"