Indicate the required fields in Autofill local card editor

Before this patch, the Autofill local card editor indicated that card
number and expiration date were required only through disabled [Save]
button, when those fields were empty.

This patch appends asterisk `*` to the end of those fields' labels and
adds a `* Required fields` label at the top of the Autofill local card
editor form.

After this patch, the Autofill local card editor more clearly indicates
that card number and expiration date are required.

Screenshot: http://screen/4VQX54VfKdgkWeu.png (Google internal)
Bug: 401294326
Change-Id: Iec310987c4c9845d672f4e388edf6d5ef071ffb9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6648084
Reviewed-by: Slobodan Pejic <slobodan@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1474961}