Support a different hostname for smartcard authentication

What does this MR do?

This MR contains proof of concept for using a different hostname for smartcard authentication. For more details please see #10526 (closed)

The limitation of the proposed solution in this MR is that the hostname for smartcard authentication MUST be a subdomain of GitLab host, e.g. cac.gitlab.example.com.

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

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

Security

This MR sets the domain attribute of _gitlab_session Cookie to include the GitLab domain and its subdomains, e.g. .gitlab.example.com

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

Closes #10526 (closed)