Add support for Cloud Functions for Firebase

[jhuleatt]

Code sample

function Calculator() {
  const functions = useFunctions();
  const remoteCalculator = httpsCallable(functions, 'calculate');

  const [calculationResult, setResult] = useState(null);

  async function handleButtonClick(firstNumber, secondNumber, operator) {
    const remoteCalculatorResponse = await remoteCalculator({ firstNumber, secondNumber, operator });
    setResult(remoteCalculatorResponse.data);
  }

  if (!calculationResult) {
    return <button onClick={() => handleButtonClick(1, 2, '+')}>Click me to add 1 + 2</button>;
  } else {
    return <pre>{calculationResult}</pre>;
  }
}

---

fixes #330

[jhuleatt]

~Tests are failing because I need to update the workflow yaml to run npm install in the functions directory~

Tests are passing now!