ENH: Add option to HttpContextBuilder to set proxy, sslContext, sslParameters, authenticator and priority.

[rbygrave]

Before this the expectation would be to set a HttpClient via HttpContextBuilder.withClient(). With this we can alternatively set each option individually on HttpContextBuilder.

  ProxySelector proxy;
  SSLContext sslContext;
  SSLParameters sslParameters;
  Authenticator authenticator;
  int priority;