[iOS] Added the model-led zero-state suggestions service

This change introduces the zero-state suggestions service, which is
responsible for asynchronously fetching the suggestions based on the
page context by querying a nano LLM. This service will be used to
provide users with contextual prompts to appear in the Gemini overlay.

Bug: 448431873
Change-Id: If162919378e3f6790fb6977f5b719705eff79cf1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7030512
Reviewed-by: Adam Arcaro <adamta@google.com>
Commit-Queue: Tarek Makkouk <tarekmakkouk@google.com>
Reviewed-by: Nicolas MacBeth <nicolasmacbeth@google.com>
Reviewed-by: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1530434}