Implement Signed Exchange reporting for distributors

This CL implements Signed Exchange reporting for distributors.
Spec: https://github.com/WICG/webpackage/pull/374

This CL has two success test cases only.
I will add more failure test cases in another CL.

Bug: 910516
Change-Id: I31a6e629073970e03a93d2379be1ae63ef6adf6b
Reviewed-on: https://chromium-review.googlesource.com/c/1468855
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634161}
22 files changed