Move Invoke to fn_go

[denismakogon]

Here's the thing, Invoke API (https://github.com/fnproject/cli/blob/master/client/invoke.go#L45) suppose to live in fn_go because it doesn't seem to be the right place for this code since Invoke API is not CLI-specific, rather than SDK-related (fn_go in particular).

As a developer, I'd like to use Invoke API to call a function with authorization using fn_go and a provider interface, but I have to use both CLI and fn_go as dependencies in order to keep the amount of code duplication low.