Implement the autofill private API functions for Autofill with AI

These functions can be used to add, update, remove, and load entities.

This API is meant to eventually replace the User Annotations API, which
was used so far by "Autofill with AI" in settings, and which is now
deprecated.

Change-Id: Ibee8952acbfa0e8a75bf0de5755844c62fef91dd
Bug: 393318055
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6214214
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Justin Lulejian <jlulejian@chromium.org>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Reviewed-by: Bruno Braga <brunobraga@google.com>
Commit-Queue: Theo Cristea <theocristea@google.com>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1423361}