Don't assume support for VK_KHR_bind_memory2,
enable the extension, if supported, during device creation.

Also initialize all required extension functions for the
memory allocator.

Bug: angleproject:4966
Change-Id: I878939fb3324723675a2b7dec18ab453b2a9fc77
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2359529
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>