[ios] Add ChromeBrowserStateManagerObserver API

To support asynchronous loading ChromeBrowserState, it is necessary
to expose a facility for code wanting to know about all instances of
ChromeBrowserState to be notified when they are loaded from disk.

Add ChromeBrowserStateManagerObserver API with methods invoked when
the ChromeBrowserState instances are created and fully loaded (can
happen after a delay when loading asynchronously).

Add unit tests to ChromeBrowserStateManagerImpl checking that the
observers are called as expected.

Bug: 335627080
Change-Id: I77cfeab5cde7d0164b26f2911b33b8238e84bd17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5752349
Commit-Queue: Aliona Dangla <alionadangla@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Aliona Dangla <alionadangla@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1335945}