[Extensions WebIDL] Support required callbacks using extended attributes

This CL changes the WebIDL processor to mark returns_async fields as
optional by default and allows them to be marked as required through the
use of a new extended attribute 'requiredCallback'. Also adjusts
existing tests to account for this and adds a new test for the new
extended attribute.

Also adds a TODO to clean some of this up in the future once we remove
the context restrictions on promise based API calls.

Background context:
Generally callbacks are inheriently optional when calling an extension
API function, as not including the callback will cause the function to
instead return a promise for any asynchronous response. However we still
carry the 'optionality' of the callback (returns_async) through the
bindings for either 1) callers from contexts which do not support
promise based calls (non-MV3/WebUI contexts) or 2) specific API
functions which still do not support promise based calls (multi-
parameter callbacks, etc.).

Bug: 340297705
Change-Id: If46f43dcfb13c013133b69db5a5beeec8c4231e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6669174
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Tim <tjudkins@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1478258}