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
  }
}

[gemini-code-assist[bot]]

Summary of Changes

Hello @amalej, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves an issue where fetching Realtime Database rules (rtdb.getRules()) would fail when connected to the Firebase emulator due to missing authentication. It introduces a specialized HTTP client that automatically provides a fake 'owner' token when an emulator host is detected, enabling seamless rule retrieval in local development environments.

Highlights

• Emulator Compatibility: The DatabaseRulesClient now correctly interacts with the Realtime Database emulator by applying a fake 'owner' credential when the FIREBASE_DATABASE_EMULATOR_HOST environment variable is detected.
• New HTTP Client for Rules: A new DatabaseRulesHttpClient class has been introduced, extending AuthorizedHttpClient, to specifically handle the authentication logic for fetching database rules, especially in emulator environments.
• Conditional Token Provisioning: The getToken() method in DatabaseRulesHttpClient is overridden to conditionally return a hardcoded 'owner' token when running against an emulator, bypassing the need for actual credentials in development.

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