Add admin users serializer and entity

What does this MR do?

In preparation for use with the migration of the Admin users table from HAML to Vue, this MR adds a new Admin::UsersSerializer and Admin::UsersEntity to collate all the necessary user values and possible actions that can be shown on the Admin users table.

There is also a broader aim to reuse some (if not all) of this work when we shift from data passed in by Rails to GraphQL: #290276

Note: This is currently not used in production but is being used in a new MR which begins the shift from HAML to Vue: !48922 (merged)

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

This could be tested by outputting the data into a HAML template or testing using the !48922 (merged) MR. Otherwise, hopefully, the tests are detailed enough 😅

• 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 #285103 (closed)