Support reading/writing categorical

[xiaodaigh]

Need

• For write, Arrow.jl to be able to generate a arrow schema from table schema

• For read, need the Arrow schema's flatbuffer definitions be readable. Maybe Arrow.jl will do that, so Parquet.jl can piggy back on that.

Update
Another possible approach is to guess what is needed, e.g. if every page in the column uses dictionary encoding then we can return that as categorical