extensions menu: Introduce ExtensionsMenuCoordinator

This CL adds `ExtensionsMenuCoordinator` for the implementation of the
extensions menu.

It is owned by `ExtensionsMenuButtonCoordinator` and
will be responsible for managing the extensions menu.

Bug: 426365719
Change-Id: I1f629b9f7357315be5cb09436321407ce5db2d3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6656142
Reviewed-by: Patrick Noland <pnoland@chromium.org>
Commit-Queue: Masa Fujita <massan@google.com>
Reviewed-by: Shuhei Takahashi <nya@chromium.org>
Auto-Submit: Masa Fujita <massan@google.com>
Cr-Commit-Position: refs/heads/main@{#1481981}