All the functionality needed for this extension has already been
implemented as part of core GLES31. Hook into that along with
changes to validation layer to allow non-GLES31 contexts to call
into these APIs as long as the extension is exposed.

Bug: angleproject:3570
Test: ProgramPipelineTest.GenerateProgramPipelineObjectEXT*
Change-Id: I92a61a47517c5cb9573874b2add6a744c9edb755
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2539121
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>