Value hyperparameters must be scalar

Value hyperparameters are currently defined to be scalar values only:

https://gitlab.com/datadrivendiscovery/ta3ta2-api/blob/devel/pipeline.proto#L69

This is restrictive, as there will be the need for primitives that take a list as an argument. For example, a primitive that removes columns from a dataframe would most elegantly be defined as taking a list of column names to drop as a parameter. As defined, we need to either have the primitive take a single column name as an arg and sequence multiple instances of it, or have the primitive take something like a JSON structure as a single string argument.