HTTP Request Maker's Code Generation output should attempt to incorporate library calls where appropriate

[rossmcdonald]

Currently the generated code is very simplistic in that it only adds the header values and sends the request. This doesn't make sense for many authenticated requests, where the headers may be time sensitive or not able to be re-used across requests.

As an example, if AWS authentication is enabled, the generated code (only for languages where libraries are available) should include calls to the appropriate AWS SDK for generating the auth signatures.