Generic entity query endpoint

[martastain]

Story

3rd party may need to query a list of entities using arbitrary conditions, such as "tasks with due date between {date} and {date}"
This is easy to do using a custom endpoint of an addon (simple SQL query), but not directly possible via REST or graphQL right now.

Proposed solution

query endpoint (already drafted, but only available for events ATM), allowing to query entities using a structured json filtering system similar to the one available in the enroll endpoint.

[BigRoy]

Would be great to have a test with this that shows how to e.g. query:

• All tasks assigned to user {user} with due dates between {start_date} and {end_date} that are children of folder path {folder_path}

So that one could e.g. query all my tasks for s01e01 that are due shortly.