Introduce SearchHijackingDetector for Extension Telemetry.

This change adds a new `SearchHijackingDetector` class to monitor for
potential search hijacking. It tracks omnibox search events and SERP
landings using new preferences. It provides a heuristic check method
that will be called periodically to detect discrepancies between search
counts and SERP landings. If a heuristic match is detected, a
`SearchHijackingSignal` is generated and stored in preferences. This
signal can be included in Extension Telemetry reports. The
`ExtensionTelemetryReportRequest` proto is updated to include the new
`SearchHijackingSignal` message. Unit tests are added for the new
detector.

The `SearchHijackingDetector` class will be owned by the
`ExtensionTelemetryService`. Instantiation of the
`SearchHijackingDetector` and invocation of its methods by the service
will be implemented in a follow-on CL.

Bug: 437345485
Skip-Clang-Tidy-Checks: modernize-use-equals-default
Change-Id: I4cce3e430874a5b588ef3629ab6f4016bbe22f1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6841820
Reviewed-by: thefrog <thefrog@chromium.org>
Commit-Queue: Anunoy Ghosh <anunoy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1503605}