add created at filter logic to users finder and API

changelogs/unreleased/feature-user-datetime-search-api-mysql.yml

+---
+title: Add creation time filters to user search API for admins
+merge_request: 12682
+author:

doc/api/users.md

@@ -146,6 +146,12 @@ GET /users?extern_uid=1234567&provider=github
 
 You can search for users who are external with: `/users?external=true`
 
+You can search users by creation date time range with:
+
+```
+GET /users?created_before=2001-01-02T00:00:00.060Z&created_after=1999-01-02T00:00:00.060
+```
+
 ## Single user
 
 Get a single user.