Limited support for running execution plugins

[DarthFennec]

There doesn't seem to be a way to specify which http method to use when calling an execution plugin. It just uses POST, even though some plugins may be set up to only use GET or something.

Also, there doesn't seem to be a way to attach a request body to an execution plugin, so I can't use this library to call plugins that take a body parameter.

[edwardl50]

I'm also facing the same issues when writing groovy unit tests, it would be nice to have support to be able to change to different REST calls