[Privacy Sandbox] Support enrollment overrides in chrome://flags

3rd party developers will need to test how their sites behave with
Privacy Sandbox APIs. However, sites are only allowed to use the APIs if
they have been enrolled/have agreed to the Privacy Sandbox attestations.
In order to unblock the developers if their site hasn't been enrolled,
this CL will let them to set a list of origins in their Chromium
instance that can use APIs (such as FLEDGE and Shared Storage) without
needing to check for enrollment.

Note that this approach is temporary. The long-term goal is to integrate
this feature into an upcoming Privacy Sandbox devtools extension. Until
the details of the extension integration have been hashed out, we are
implementing this using a flag.

Change-Id: If742873024c9ea2e450ac6ca827bf07de1a5fd92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4638542


Reviewed-by: Garrett Tanzer <gtanzer@chromium.org>
Commit-Queue: Liam Brady <lbrady@google.com>
Cr-Commit-Position: refs/heads/main@{#1161847}