feat(cloud_functions): expose allowInsecureTokenAttachment for Apple debug builds

[SelaseKay]

Description

Exposes FirebaseFunctions.allowInsecureTokenAttachment so a physical Apple device can send Auth, FCM, and App Check tokens to a Functions emulator on a LAN address.

Since Apple SDK 12.17, Functions refuses those tokens over plain HTTP unless the host is loopback. A device cannot use the Mac's loopback, so useFunctionsEmulator('192.168.1.x', 5001) fails with unauthenticated when App Check is enforced. SDK 12.19.0, already pinned by firebase_core, adds a debug-only opt-in. This PR forwards that flag from Dart through call() and stream(), and sets it on the Apple Functions instance inside #if DEBUG. Android and web already attach the tokens, so the flag has no effect there. Profile and release builds ignore it because the Apple SDK does not compile the property in.

Related Issues

Fixes #18732

Related to #18714

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

Test plan

• flutter test in cloud_functions (firebase_functions_test.dart)
• flutter test in cloud_functions_platform_interface (method_channel_https_callable_test.dart)
• dart analyze on cloud_functions and cloud_functions_platform_interface
• Debug flutter run on a physical iOS device against a Functions emulator on a LAN IP with App Check enforced

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