[FR][Database] Support connection to local Firebase emulator (Android/iOS/desktop)

### [REQUIRED] Please fill in the following fields:

* Unity editor version: 2020.1.9 personal
* Firebase Unity SDK version: 6.15.2
* Source you installed the SDK: 6.15.2 (.unitypackage or Unity Package Manager)
* Problematic Firebase Component: firebase databse realtime (Auth, Database, etc.)
* Other Firebase Components in use: firestore, functions(Auth, Database, etc.)
* Additional SDKs you are using: none (Facebook, AdMob, etc.)
* Platform you are using the Unity editor on: windows (Mac, Windows, or Linux)
* Platform you are targeting: iOS, Android, desktop (iOS, Android, and/or desktop)
* Scripting Runtime: Mono (Mono, and/or IL2CPP)

### [REQUIRED] Please describe the issue here:

(Please list the full steps to reproduce the issue. Include device logs, Unity logs, and stack traces if available.)

#### Steps to reproduce:
Hi, I'm currently having a problem with the value change listening event in Unity. I use Firebase emulator locally. I just get the snapshot with null value. Can you help me solve this problem. Thanks.


This code is working when I use firebase database realtime on cloud.

The issue does not name a source file, test, or entry point. Start by locating the Unity Realtime Database quickstart and its local Firebase emulator usage, then reproduce the value-change listener scenario with the stated SDK and platforms. Done means the emulator connection behavior is supported or the limitation is clearly documented and covered by an appropriate test.

Written by the indexing model from the issue text.