feat(firebase_app_check,windows): add custom App Check provider support

[SelaseKay]

Description

Adds custom App Check provider support for Windows, addressing the custom-provider part of #12227. Combined with the existing Windows debug provider, this lets Flutter Windows apps meet App Check enforcement for Firestore, Functions, Storage, Auth, and Realtime Database.

Previously, Windows activate() always installed the C++ debug provider. Apps can now pass WindowsCustomProvider(fetchToken: ...) so the native plugin asks Dart for a minted token (and its expiry) whenever the Firebase C++ SDK needs a fresh App Check token.

Usage:

await FirebaseAppCheck.instance.activate(
  providerWindows: WindowsCustomProvider(
    fetchToken: () async {
      final response = await myBackend.mintAppCheckToken();
      return CustomAppCheckToken(
        token: response.token,
        expireTimeMillis: response.expireTimeMillis,
      );
    },
  ),
);

fetchToken is invoked by the plugin when the native SDK needs a token. App code never touches generated Pigeon FirebaseAppCheckFlutterApi; a private method-channel implementation registers that callback per app.name. The native factory is process-global and long-lived, with per-app custom vs debug dispatch at GetToken time.

This reimplements closed #18184 on current main (including recaptchaSiteKey on activate) and incorporates the review feedback from that PR.

Related Issues

#12227

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• 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.

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