fix: resolved wrong pagination info (#17)

This PR contains a fix made on pagination during the retrieve of the mock resources list. The fix provide the execution of a dedicated query for retrieve the total number of elements in the whole collection, resolving a bug that caused a wrong value for the count of elements in page.

#### List of Changes
 - Refactored query for get the count of all elements
 - Added new field for retrieve the total number of elements
 - Added `@Indexed` annotation on mock resource entity