Add APIs to support scaled egress testing; custom directory sync; and automated admin tools

[Sing-Li]

Extend REST APIs to:

1. support large scale egress tests.. like this...
   
   15,000 users in ...
   
   5,000 rooms
2. provide the 'Rocket.Chat' half of the equation for custom implementations of directory synchronization (such as AD Sync in C#, or LDAP bulk import via JNDI in Java)
3. enable the creation of automated administration tools