Allow Fission functions to be dynamic tasks

[erwinvaneyk]

Currently only internal functions can be dynamic tasks (tasks that return a task). Support should be added to the fission runtime environment to also allow fission functions to be dynamic tasks.

One approach would be that in the presence of a special header X-Fission-Workflow-Dynamic the body is interpreted as a task. Or, maybe even better, having the content-type be something like application/vnd+fission.workflow+json. By specifically requiring the header we avoid outputs to be accidentally being interpreted as dynamic tasks.