Use project depended feature flag for pages ssl and get certificates only for enabled domains

What does this MR do?

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/28996

We already have pages_auto_ssl feature flag for hiding gitlab instance level admin settings.

This MR adds another feature flag enabling us to selectively enable auto ssl support for projects on production

Also add guard close to prevent obtaining certificates for disabled domains

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