Experimental add direct host

[KazuCocoa]

This is an experimental feature to improve requests peformance in a particular case.

If a server returns directConnectProtocol, directConnectHost, directConnectPort and directConnectPath as a part of the response of create session, Ruby client send following requests to them.
This feature improves performance if a user handles requests via proxy server between clients to Appium servers. In this context, the proxy server is similar to SeleniumGrid.

# via proxy server in session creation
client <--> proxy server <---> appium server <-> devices
                                             `-> appium server <-> devices

# communicate directly after the session creation
client <--------> appium server <-> devices
                   `---> appium server <-> devices

This idea was discussed with @jlipps

[KazuCocoa]

https://github.com/projectxyzio/web2driver#direct-connect-urls