Add OncallScheduleEntity to users in Admin helper

What does this MR do?

This adds Oncall Schedule information into the users data when rendering in the admin/users page.

We use this to warn that the user is part of an Oncall Schedule before they are removed. See #323629 (closed) for more details.

Note, this is only rendered via the vue_admin_users Feature is enabled. It's currently scheduled to be enabled in %13.11 #290737 (closed).

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because no user facing changes until implemented by frontend.
• 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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 #323629 (closed)