Addition of buffer support for external memory
Also adds new end2end tests for these usecases
  * SubData update
  * map/unmap buffer
  * dispatch compute with external buffer

Bug: angleproject:5073
Test: ExternalBufferTestES31.*Vulkan
Change-Id: Ib3cccaca77b76830effe49d3731782552e7424ec
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2525105
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>