[FEATURE] Output transformer

[goodwanghan]

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

Describe the solution you'd like
Currently Fugue transformers must return something. Sometimes we just want to distributedly process something and then stop. For example, for a big dataframe we can partition by certain keys, then we can use a transformer to serialize and save each partition (in its own way).

For the current design, we need to transform and persist to trigger this execution. It works fine but concept wise, it's a hack. We need to have a clear definition of this as a new type of extension, maybe OutputTransformer?

And we need to add programming and sql interface, and interfaceless for it. Also update the doc.

Describe alternatives you've considered
NA

Additional context
NA