feat: add Firebase Cloud Functions plugin

[robingenz]

Pull request checklist

Please check if your PR fulfills the following requirements:

• The changes have been tested successfully.
• A changeset has been created (npm run changeset).
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Close #640

Firebase docs:

• https://firebase.google.com/docs/reference/android/com/google/firebase/functions/HttpsCallableReference#call()
• https://firebase.google.com/docs/reference/swift/firebasefunctions/api/reference/Classes/HTTPSCallable#call_:

[robingenz]

npm i @capacitor-firebase/functions@6.0.0-dev.b96fa6c.1718781196