Full proxy support to configuration

[yunhaoling]

For now, we have a parameter called 'http_proxy' to configure proxy when initializing the EventHub Client.

It's a dictionary object with the following keys:

keys: 'proxy_hostname' (str value) and 'proxy_port' (int value).
Additionally the following keys may also be present: 'username', 'password'.

[yunhaoling]

This is done, we decide to keep the http_proxy kwargs.