Support headers (and other metadata) as input to workflow

[erwinvaneyk]

Currently the only supported input to a Fission workflow is the body. All other aspects of the (http) request are ignored.

To conform workflows to the (expected) consistent behavior of Fission functions, we need to add at least the following:

• [x] headers (this automatically also include the Fission-specific headers that are included everywhere)
• [x] query
• [x] other metadata (e.g. http method)