Rebuild absent embeddings from source passages to support model changes

This CL schedules embeddings to be recomputed for any rows in the
passages table that don't have corresponding rows in the embeddings
table, keyed on url_id. This rebuild process happens once when the SQL
database is initialized, and partial progress can lead to full recovery
even when interrupted by shutdown. Rebuilds can be disabled by forcing
the "RebuildEmbeddings" feature parameter to false, and model version
changes can be simulated to delete the embeddings data by forcing
the "DeleteEmbeddings" feature parameter to true. Models can change
any time, even without deploying a new Chromium binary, and without
this CL the history data would not be available for embeddings search,
but with this CL the history data gets rebuilt to become searchable.

Bug: 345798663
Change-Id: I46a45d5d8d55ac6a38b70df985044b610f23abef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5606010


Reviewed-by: John Lee <johntlee@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Orin Jaworski <orinj@chromium.org>
Reviewed-by: Orin Jaworski <orinj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312838}