Addition of support for immutable storage to buffer objects.
Also adds new end2end tests for these usecases
 * Basic BufferStorage
 * SubData update
 * map/unmap buffer

Bug: angleproject:5056
Tests: angle_end2end_tests --gtest_filter=BufferStorageTestES3*Vulkan
Change-Id: Iba74b372ad033711927b63c6a04cec0eeb4db699
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2419952Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>