GL_OES_EGL_image_external_essl3 support was added a while back.
This patch just enables it for the Vulkan backend

Bug: angleproject:2668
Test: ./angle_end2end_tests --gtest_filter=ImageTestES3*Vulkan
Change-Id: I6b8b1f63ed6e9faafe668ad9a73fc7fbc9c17670
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1852190
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>