Formalize function call API

[erwinvaneyk]

Currently Fission Workflows relies directly on the internal executor API to execute functions. We want to avoid this usage of the internal APIs, as these executor and other APIs should not be dependent on.

For this reason we need a clear function call API (which in some sense now exists implicitly in the fission-function/* path of the router). In the future this API could be used to call a specific version of a function in order to make workflows more robust to frequently changing functions.

cc @soamvasani

[erwinvaneyk]

Wrong repo, see fission/fission#632