Add domain verification operation

Related issue #375492 (closed)

What does this MR do and why?

Add domain verification operation actions in group level.

Add a development feature flag to control: domain_verification_operation

The controller actions are almost same to those in app/controllers/projects/pages_domains_controller.rb. Reuse the PagesDomain model and services, the main difference being the project_id parameter in create action.

frontend part MR !116647 (merged)

dependent MR !116621 (merged)

splited from MR !115766 (closed)

/cc @daveliu

Screenshots or screen recordings

How to set up and validate locally

1. simulate the SaaS locally:

   export GITLAB_SIMULATE_SAAS=1

2. Upload Premium license to enable domain_verification feature

3. enable domain_verification_operation feature in console:

   Feature.enable(:domain_verification_operation)

4. Visit new domain verification page http://localhost:3000/groups/flightjs/-/settings/domain_verification/new

MR acceptance checklist

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