Add Support for Environment Proxy Configurations

[alzimmermsft]

Updated the Netty and OkHttp HttpClients to check the environment for proxy configuration if the ProxyOptions set in their builder is null. Additionally, added the ability to set the Configuration object that the builders will use when retrieving proxy information from the environment.

Environment proxy configurations will be loaded in the following order:

1. Azure HTTPS
2. Azure HTTP
3. Java HTTPS
4. Java HTTP

Azure proxy configurations are preferred over the Java proxy configurations as they relate more closely to the SDKs, additionally HTTPS is preferred over HTTP as it is more secure. Java proxies will require java.net.useSystemProxies to be true before that proxy configuration is applied.

[alzimmermsft]

@srnagar I replied to your comments if you want to do another quick pass.