Create Ci::RunnerMachine model

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds the ci_runner_machines table according to https://docs.gitlab.com/ee/architecture/blueprints/runner_tokens/#runner-identification-in-ci-jobs.

Closes Create database migration to add `ci_runner_mac... (#386493 - closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Go to the shell in your GDK gitlab directory and run bundle exec rake "gitlab:seed:runner_fleet". This will seed your GDK with some runners and jobs required for testing this MR;

2. Run the following code on the GDK Rails console:

   Ci::RunnerMachine.new(runner: Ci::Runner.last, machine_xid: 's_test', executor_type: 'docker').save!
   Ci::Runner.last.runner_machines

   This should create a new ci_runner_machines records and associate it with the last runner.

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.