[FEATURE] Container Image / Alternative Language Steps

**Is your feature request related to a problem? Please describe.**

Not really a problem, but the flexability to use other runtimes would be an interesting one for a few use-cases; such as image manipulation, binary protocols, and machine learning.

**Describe the solution you'd like**

In addition to "NodeJS Step", it would be rather cool to use other languages or container images with pre-built functions. The OpenFaaS project has a selection of images that provide functionality through the use of their fwatchdog binary that translates HTTP requests into stdin, and stdout into HTTP responses. I suspect Pipedream could use a similiar approach for chaining steps.

OpenFaaS watchdog:

https://docs.openfaas.com/architecture/watchdog/

A list of the OpenFaaS functions:

https://hub.docker.com/u/functions