fix(rtdb): Apply fake credentials when getting rules for rtdb emulator

[amalej]

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Fixes #2034

Running the ff code:

import { initializeApp, cert } from "firebase-admin/app";
import { getDatabase } from "firebase-admin/database";

process.env.FIREBASE_DATABASE_EMULATOR_HOST = "127.0.0.1:9000";

const admin = initializeApp({
  projectId: "demo-project",
  databaseURL: "http://127.0.0.1:9000/?ns=demo-project-default-rtdb",
});
const rtdb = getDatabase(admin);

async function main() {
  const rules = await rtdb.getRules();
  console.log(rules);
}

main();

Outputs

>node .
{
  "rules": {
    ".read": true, // Message from rules json
    ".write": false
  }
}