Added Endpoint for Challenge Assignments

Our first endpoint for Challenges!

You can now retrieve all assignments for a specific challenge (using a challenge ID), and with it, grading and completion data.

Check it out: https://developers.workramp.com/reference/get-challenge-assignments

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Please note that the time_spent value (calculated in seconds), is updated once daily. Until that update happens, you will see a null value returned. You will need to pull the value again after the daily update/sync.

Please note that the time_spent value (calculated in seconds), is updated once daily. Until that update happens, you will see a null value returned. You will need to pull the value again after the daily update/sync.

Please note that the `time_spent` value (calculated in seconds), is updated once daily. Until that update happens, you will see a `null` value returned. You will need to pull the value again after the daily update/sync.