Add json IO mappers

[MaksymDel]

These are unique mappers since they read/write data from/to disc, so I made a new file for them.

I think they can provide three primary benefits to the users:

1. They allow reading/writing files in parallel with HF Dataset
2. They allow the creation of end2end data workflows with Pipeline objects.
3. They make users aware that this is something they can do, e.g., with their more complex Reader/Writer classes.