[iOS][BWG] BWG tab helper and currently active state

BWG tab helper which controls the feature for a given tab. Observe's the
associated WebState's actions such as `Shown` and `Hidden`, and for now
includes simple logic of showing/hiding of the BWG UI, but soon will
hold more persistence logic.

Bug: 419070203
Change-Id: Ifc6320681502fc2d0948bfecc6ad6febe18ddabf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6681289
Reviewed-by: Adam Arcaro <adamta@google.com>
Commit-Queue: Nicolas MacBeth <nicolasmacbeth@google.com>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1480076}