[Unencoded-Digest] Parse `unencoded-digest` into `URLResponseHead`.

This CL adds a call to parse `unencoded-digest` during the execution of
`url_loader_util::BuildResponseHead()`, and adds a corresponding
property to the `URLResponseHead` mojo struct.

This is one of a chain of CLs moving `unencoded-digest` parsing and
error reporting to the network service:

1. https://crbug.com/c/6734707 (Introduces the header's parser.)
2. https://crrev.com/c/6734019 [You are here.]
3. https://crrev.com/c/6771890 (DevTools integration)
4. https://crrev.com/c/6798148 (Wire up enforcement in Blink)

Bug: 431760410
Change-Id: Ib7498890da08f7cbd35cb6009db22dfb4797850c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6734019
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1494679}