client conversion of schema to handler

[cpritcha]

Handlers support particular resource types and depend on their source library (for example pandas in Python and dplyr in R). Different libraries support different resource types so it may be that the resource classes supported by one model as input are not supported by another library.

An interactive function to list the handlers compatible with a particular schema would help model users try out a new model in the REPL.

Something like

compatible_handlers(schema)
> PandasHandler
>  matches with:
>    - Feather
>    - Parquet
>
> SparkHandler
>  matches with:
>    - Parquet

that shows the resource types supported by different handlers