Extract MacOS runners environment configuration to a dedicated module

Extract MacOS runners environment configuration to a dedicated module under config-mgmt/modules/ci-runners

Remember to cover:

• licenses management
• autoscaling groups for runner managers
• S3 cache bucket
• SSH key to access the instances
• Service account (+credentials) for the runner to use API
• Service account (+credentials) for the runner to upload cache into the dedicated S3 bucket

Will be done with https://ops.gitlab.net/gitlab-com/gl-infra/config-mgmt/-/merge_requests/5308.

Once done it will also handle #42 (closed)