Addition of the entry points for EXT_separate_shader_objects extension.

Bug: angleproject:3570
Change-Id: I95a1342bb3322b49997125a17367590183134c84
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2539120
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>