Support for Token Server into the plugin

[jalamprea]

To simplify the process to use the plugin for WP Users, this plugin will embed its own Token Server.
This Token Server should be implemented based on the Agora SDK Docs:
https://docs.agora.io/en/Audio%20Broadcast/token_server_php?platform=PHP

This plugin should expose the Token Server URL directly from the plugin.
This embebed server will help users to avoid the creation of custom token server and they will only need to setup the certificate ID from Agora on the WP Settings.

This task is blocking #6