[Trigger Attestation] Add trigger_attestation to url_response head

Adds a structure containing a token that can be sent to a reporting
origin alongside an aggregatable report for it to attest the validity
of a trigger. The token is signed over the included
`aggregatable_report_id` which must be used to create the aggregatable
report.

It is added as a property on the response to transfer the token from the
network service (where it's generated) to blink where it is transferred
to the browser alongside of the attribution data that it attests.

It will be possible to remove this property once crbug.com/1374121 is
resolved (i.e. when attribution response parsing moves to //net).

Bug: 1405832
Change-Id: I5e79d64dbf34b88b0f04e4b2c57ef8b1f624d4ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4204010
Reviewed-by: Ian Vollick <vollick@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: Anthony Garant <anthonygarant@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1102838}