Create virtual host configuration API for GitLab Pages

What does this MR do?

This MR creates internal API that GitLab Pages can use to request virtual host configuration for given host.

There are two major parts:

• Authentication between the Pages and the Rails app (using JWT similar to how GitLab Workhorse and the Rails app talk)
• Internal API endpoint GET /pages?host=... that returns the virtual host configuration for this domain

Port to EE MR - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14821.

Omnibus MR - omnibus-gitlab!3444 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• 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

Related to #61927 (moved).