[Autofill] Import formatted dates into EntityInstances

This CL adds importing of formatted dates into EntityInstances.
It only handles format strings for numeric dates. It does not handle
<select> elements for named months yet.

Only complete dates (year, month, day) are imported. Dates may be
spread over multiple fields.

Bug: 396325496
Change-Id: Ife59c2384b9d9baa96f615c0d6de0a8715376166
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6320533
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1428756}