feat(appcheck): appcheck recaptcha enterprise support

[cynthiajoan]

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[cynthiajoan]

/gemini review

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Code Review

This pull request introduces support for reCAPTCHA providers across Android, iOS, and Web platforms in firebase_app_check and integrates recaptchaSiteKey into FirebaseOptions across firebase_core platforms. Key feedback includes removing mavenLocal() from build files to prevent consumer dependency issues, adhering to the repository style guide by avoiding the ! operator in _safeToDart, and exporting WebProvider to replace dynamic types with safer, strongly-typed declarations in the example app.