Update invokefunction to return headers

[alexellis]

Task:

• Update invokefunction to return the response headers
• Update the caller function named Invoke to return the body, statusCode and errors
• Update example to print statusCode and body

Suggested by Simon Pezcler and Rishabh Gupta so that connectors can do better debug and potentially use the result for something.

Right now the result is abandoned.

[zeerorg]

We can use headers for logging in response to function invocations, it can be especially useful when connecting to outside services since.

[zeerorg]

This issue should be closed.

[Supesharisuto]

is this the extra header params in https://github.com/openfaas/connector-sdk/blob/d791db5110e29164b697cb133d39f40812296f01/types/controller.go#L58