[HPRT Sample] Amend URT lookup mechanism to send background HPRT lookups

This CL adds a new parameter to the UrlRealTimeMechanism. This parameter
enables the mechanism to trigger both a UrlRealTimeMechanism and a
background HashRealTimeMechanism if the latter mechanism is available.
A later CL will compare their results to determine whether a CSBRR
should be sent for the hash real-time lookup. The results processing
code is not yet implemented and will be addressed in a separate CL.

Bug: 359609447
Change-Id: I5baeb29c8f287d2525c43dcaaf98918aa8d43a65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5800153
Reviewed-by: thefrog <thefrog@chromium.org>
Auto-Submit: Zack Han <zackhan@chromium.org>
Commit-Queue: Zack Han <zackhan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1348871}