PhysicalFileProvider: Use active polling instead of FileSystemWatcher on iOS/tvOS...

PhysicalFileProvider: Use active polling instead of FileSystemWatcher on iOS/tvOS (#58165)

In https://github.com/dotnet/runtime/issues/57931 we found out that the FSEventStream APIs which are used to implement System.IO.FileSystemWatcher aren't allowed on the App Store.
According to [Apple's docs](https://developer.apple.com/documentation/coreservices/1443980-fseventstreamcreate) these APIs are only supported on macOS and Mac Catalyst.

Mark System.IO.FileSystemWatcher as unsupported on iOS/tvOS and throw PNSE.
Switch PhysicalFileProvider to use active polling instead, like we do for Browser.

Addresses https://github.com/dotnet/runtime/issues/57931

Co-authored-by: Alexander Köplinger <alex.koeplinger@outlook.com>