[API] Add created_at and updated_at key/value pairs for GET .../conferences API call

[Vitalii]

Problem statement:

The GET request "/api/v1/courses/:course_id/conferences" doesn't return "created_at" and "updated_at" timestamps which is very inconvenient in my case, — for only getting these two fields I need to make a long and bulky Canvas Data 2 request for web_conferences table.

Proposed solution:

Add "created_at" and "updated_at" to the conferences API call response.

User role(s):

admin