Wrap Firestore Remote Config's addOnConfigUpdateListener method for use with AsyncThrowingStream.

RemoteConfig+addOnConfigUpdateListener.swift

import Foundation
import FirebaseRemoteConfig

extension RemoteConfig {
    func addOnConfigUpdateListener() -> AsyncThrowingStream<Set<String>, Error> {
        .init { continuation in
            addOnConfigUpdateListener { configUpdate, error in
                if let error {
                    continuation.finish(throwing: error)
                } else {
                    continuation.yield(configUpdate!.updatedKeys)
                }
            }
        }
    }
}