Add sampler object to back end and implement creation and info query.

Bug: angleproject:6001
Change-Id: If5b7d09564d773d6aeda7612728e19bfec7f6155
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2912806
Commit-Queue: John Plate <jplate@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>