feat(enterprise/coderd): allow system users to be added to groups (#18341)

closes https://github.com/coder/coder/issues/18274

This pull request makes system users visible in various group related
queries so that they can be added to and removed from groups. This
allows system user quotas to be configured. System users are still
ignored in certain queries, such as when license seat consumption is
determined.

This pull request further ensures the existence of a
"coder_prebuilt_workspaces" group in any organization that needs
prebuilt workspaces

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
  * Organization and group member listings now include system users.
* **Bug Fixes**
* Updated tests to reflect the inclusion of system users in member and
group queries.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

coderd/database/queries/organizationmembers.sql

@@ -89,6 +89,8 @@ WHERE
 			organization_id = @organization_id
 		ELSE true
 	END
+  -- Filter by system type
+	AND CASE WHEN @include_system::bool THEN TRUE ELSE is_system = false END
 ORDER BY
 	-- Deterministic and consistent ordering of all users. This is to ensure consistent pagination.
 	LOWER(username) ASC OFFSET @offset_opt