Add skeleton Pages internal API

What does this MR do?

This MR is extracted from https://gitlab.com/gitlab-org/gitlab-ce/issues/61927 in order to break the later in smaller chunks.

It implements basic /internal/pages endpoint that will be used for the Pages virtual domain internal API. The endpoint is currently behind feature flag and provides authetication similar to how Workhorse is authenticating with GitLab.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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)