Populate Autofill form/field signatures + global IDs via extension API

The extension API already allows querying an optimization_guide::proto::
FormData from Autofill's cache. This CL populates some additional fields
in that proto that were added in cl/738001218 / crrev.com/c/6369199.

Since the proto is also used to make model requests, and since the new
fields shouldn't be populated in that case, this CL adds a "conversion
reason" parameter to the proto conversion util.

Bug: 389631485, 395038288
Change-Id: I17793ad737075996018c0d2feba03844fbce1150
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6368555
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1435307}