[Extensions] Attach extensions menu platform delegate to model

Adds method that attaches the menu platform delegate to a platform-agnostic menu model. This will be needed for UI interaction callbacks
to be redirected to the model, which handles extension logic changes.

Bypass-Check-License: no new or moved file, unsure why presubmit fails
Bug: 449814184
Change-Id: I9937e2844821789f826b7eed52211038d0695a45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7023603
Reviewed-by: Shuhei Takahashi <nya@chromium.org>
Commit-Queue: Emilia Paz <emiliapaz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1529664}