Fix bug where shorthand notation and parameterized service account didn't work for functions

[taeold]

The change allows developers to set service account configuration using short notation (e.g. my-sa@) and using string parameters:

import { onRequest } from "firebase-functions/https";
import { defineString } from "firebase-functions/params";
import * as logger from "firebase-functions/logger";

const sa = defineString("SERVICE_ACCOUNT")

export const helloSAParams = onRequest(
  { serviceAccount: sa },
  (request, response) => {
    logger.info("Hello logs!", { structuredData: true });
    response.send("Hello from Firebase!");
  });

export const helloShorthand = onRequest(
  { serviceAccount: "firebase-app-hosting-compute@" },
  (request, response) => {
    logger.info("Hello logs!", { structuredData: true });
    response.send("Hello from Firebase!");
  });

fixes #5224, #8227