[Extensions] Handle find alternative button in mv2 deprecation panel

An extension's 'find alternative' button in the mv2 deprecation panel is
visible iff the extension has a recommendations urls, which only exists
if the extension has a webstore url.

Clicking on the button opens a Web Store URL with extension
recommendations related to such extension.

Screencast: https://drive.google.com/file/d/1-McBunXWl-_4dZWBzH9IYkCVkpsauBon/view?usp=sharing

Bug: 337191307
Change-Id: I6ecde791b137278008ce813035e38e7a9f3ee05c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5530868
Commit-Queue: Emilia Paz <emiliapaz@chromium.org>
Reviewed-by: Kelvin Jiang <kelvinjiang@chromium.org>
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Auto-Submit: Emilia Paz <emiliapaz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1300281}