Writing Assistance APIs: Add static availability and create functions

Align create() and availability() with updated explainer/spec:
  https://github.com/webmachinelearning/writing-assistance-apis

Update IDL, remove *Factory classes and `ai` namespace accessors.
Add AIInterfaceProxy::GetAIManagerRemote util function.

Consolidate availability() & create() impls on base template classes.
Add Writer/Rewriter AICreateMonitor support via IDL & *CreateClient.
Define specializations for templated helpers.

Update WPTs and global-interface-listing*.

Bug: 381969447, 405996212, 402442890
Change-Id: Iff3be641d27d3a37d9cb2cbaf9c35e977bcf1274
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6394420
Reviewed-by: Daseul Lee <dslee@chromium.org>
Commit-Queue: Mike Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1438481}