Add an instance-level serverless domain from the admin area

What does this MR do?

BE and FE of admin panel for #35591 (closed)

/cc @mikegreiling

Screenshots

New domain form	Add certificate to existing domain

Unverified domain	Verified domain

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

Note: this feature is behind the :serverless_domain feature flag.

$ rails c
pry> Feature.enable(:serverless_domain)

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team