Add AutofillAiModelCache

This CL adds a barebones version of a cache for AutofillAI model
responses. The cache is currently just a std::map and is not used
anywhere yet.

Follow-up CLs will add a persistence layer and start using it for
AutofillAI.

Bug: 389631477
Change-Id: I7e5c5aeab7a96e98c3c72e98c3ad9f476e31dbdb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6309252
Reviewed-by: Christoph Schwering <schwering@google.com>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1427016}