Addition of the entry points for EXT_external_buffer extension.

Bug: angleproject:5073
Change-Id: I5dc9f74dfd68c1e83e17ca15166df4cd4c20fca2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2525104
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>