Implement FilterExtractor logic to extract and store annotations

This change updates FilterExtractor to use AnnotationIndexClient to fetch FilterAnnotation from a given URL and then uses FilterStore to persist the extracted annotation. Unit tests are added to verify successful extraction, handling of empty results, and robustness against null dependencies.

Fixed: 483670268
Change-Id: Ic8c3bc1d4b92789650ee6a40295cc64dc0dc99f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7663439
Commit-Queue: Simge Şengün <simges@google.com>
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Cr-Commit-Position: refs/heads/main@{#1599939}