Add birch weather provider

Adds a birch item struct for weather, with some updates to the generic
BirchItem - adds an icon image model, and changes the title to u16
string.

Introduces birch weather provider, that uses ambient mode backend to
query current weather state - not using the ambient mode weather model
at this point so the network requests tags when downloading icon are
different, and eventually, the backend call will be updated to use a
different weather client ID. Also, birch currently does not use polling
for weather changes.

Still todo - check whether geolocation is enabled before requesting
weather.

BUG= b/323229328

Change-Id: I5494f0afe697be5d053b201b494a0e745d114fd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5255822
Reviewed-by: Matthew Mourgos <mmourgos@chromium.org>
Reviewed-by: Eric Sum <esum@google.com>
Commit-Queue: Toni Barzic <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1256514}

• ash/BUILD.gn[diff]
• ash/ambient/backdrop/ambient_backend_controller_impl.cc[diff]
• ash/birch/birch_item.cc[diff]
• ash/birch/birch_item.h[diff]
• ash/birch/birch_model.cc[diff]
• ash/birch/birch_model.h[diff]
• ash/birch/birch_weather_provider.cc[Added - diff]
• ash/birch/birch_weather_provider.h[Added - diff]
• ash/birch/birch_weather_provider_unittest.cc[Added - diff]
• ash/constants/ash_features.cc[diff]
• ash/constants/ash_features.h[diff]
• ash/public/cpp/ambient/ambient_backend_controller.h[diff]
• chrome/browser/ui/ash/birch/birch_keyed_service_unittest.cc[diff]
• chrome/browser/ui/ash/birch/birch_recent_tabs_provider.cc[diff]