allow using sis id in courses user fetch API

fixes CNVS-13657

test-plan:
- Have a user with a sis id
- Enroll user in a course in Canvas
- Fetch the course scoped user using the sis id (eg. <canvas>/api/v1/courses/<id>/users/sis_user_id:<sis_id>
- Verify that you receive the user's information

Change-Id: I7eeb2aaa17021a3a598f30279e7fb121b94f00ff
Reviewed-on: https://gerrit.instructure.com/36626
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>