macos: skipEntitlementsChecks()

[renanyoy]

would be nice to implements again skipEntitlementsChecks() as the checks are always done on mac and it impeach to use filepicker in a non sandboxed app. I had to make a fork, to always skip the checks...

[vicajilau]

This is actually a regression, not just a feature request. The native skipEntitlementsChecks mechanism always existed and worked in MacOSFilePickerHandler, but the public Dart entry point was turned into a deprecated no-op during the v12 federation rewrite, on the assumption that entitlement checks would be handled automatically. They are not, checkEntitlement never checks whether the host app is actually sandboxed, so a non-sandboxed app has no way to opt out today.

Opened a fix restoring a working entry point. Since this is macOS-specific, it now lives on FilePickerDarwin (package:file_picker_darwin) rather than on the generic FilePicker facade, matching how other platform-specific APIs like AndroidSAFHandle are exposed. See #2150.