[tab suggestions] Create observer and coordinator

This CL initializes the two important components of the feature:
SuggestionEventObserver which consumes tab-related events and
GroupSuggestionsPromotionCoordinator which is the UI component of the
feature that gets suggestions from backend and shows them to users.

One thing worth mentioning about this CL is that this CL changes
how hub-related components are initialized in coordinator. This is
because that coordinator should be able to created without hub-related
components being ready.

With this CL, we have one early stage e2e demo: https://drive.google.com/file/d/1TBxDTG6Hs1G0WNEz2RsvbxldQkX0urg2/view?usp=sharing&resourcekey=0-Nv27mqiND2Po8Vn2-TwL1A

Bug: 397221723
Change-Id: I0c26e82b91b7caea6cea06a9d708702cee9f68ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6380237
Reviewed-by: Siddhartha S <ssid@chromium.org>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Commit-Queue: Yue Zhang <yuezhanggg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1436301}