YAML API improvements

[erwinvaneyk]

This PR fixes usability issues when using YAML to define a workflow. To improve the usability it...

• Fixes the usage of body to input the body for a Fission function.
• In documentation promotes using body over default when inputting the body of a fission function.
• Adds body usage to all examples to clarify the usage of the body input.
• Adds a test to e2e that tests for setting the method, headers, query and body for a fission function.