Allow to cache the field classification model predictions.

This CL allows FieldClassificationModelHandler to cache and retrieve
predictions for a given classified form. Predictions are stored in
a Least Recently Used cache, and are keyed by a 64 bit hash of all
model inputs accumulated in a single string.
The behaviour is behind a new feature flag.

Bug: 420379230
Change-Id: I4ea644142cafc7b0087ed721d15d96f21cdbe984
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6586949
Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Commit-Queue: Maria Kazinova <kazinova@google.com>
Cr-Commit-Position: refs/heads/main@{#1466373}