Make ICE configuration pluggable

[atsakiridis]

Keep in mind that TURN servers return JSON, which isn't standardized. So developers need an easy way to plug in their on TURN server and support it's customized JSON

Right now we need to support xirsys and the other internal format, but in the future we should be able to use any format.

At the same time let's also support direct setup of stun/turn servers without configuration URL (similar as we do in webrtcomm)

[atsakiridis]

ognjenns I finalized a discussion about this with Jean. So here are some more details on what we need. The user should be able to initialize RCDevice in one of 3 ways as far as ICE configuration is concerned:

1. To use Xirsys V2 configuration URL. This is the only option we currently have; you can check here to see how the V2 url looks and the asynchronous call towards the configuration URL
2. To use Xirsys V3 configuration URL. The is the 1st new option where the configuration URL has different format: https://username:secret key@ice.restcomm.io/_turn/restcomm
3. To NOT use a configuration URL at all and directly pass STUN and TURN server URLS in a list. This is the 2nd new option

We can discuss more if you want when the time comes