Defer source/trigger JSON parsing to interop execution

Instead of parsing registrations during interop-test instantiation, we
defer parsing until the test is being run. This increases the testable
surface area and makes execution more closely resemble the real behavior
of the API, which parses registrations at the time they arrive.

We allow the registrations themselves to be arbitrary JSON, instead of
requiring them to be dictionaries at instantiation time, in order to
further increase the testable surface area.

Change-Id: Ie6d657998c4071167ade30d189e279434ca0b713
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4789905
Reviewed-by: Nan Lin <linnan@chromium.org>
Commit-Queue: Andrew Paseltiner <apaseltiner@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1184742}

• content/browser/attribution_reporting/attribution_interop_parser.cc[diff]
• content/browser/attribution_reporting/attribution_interop_parser.h[diff]
• content/browser/attribution_reporting/attribution_interop_parser_unittest.cc[diff]
• content/browser/attribution_reporting/attribution_interop_runner.cc[diff]