[Extensions c2s] Add permissions.addSiteAccessRequests

Allow the extension to show site access requests at runtime without any
user gesture by introducing permissions.addSiteAccessRequest. Request
will only be signaled to the user if extension can be granted access to
the site in the request.

API is only available for MV3 extensions.

Bug: 331680187
Change-Id: Ie87f9ce5c651aa2bbd15c766fca686317c96d91f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402375
Reviewed-by: Justin Lulejian <jlulejian@chromium.org>
Commit-Queue: Emilia Paz <emiliapaz@chromium.org>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1309664}