csv feature gate

[ritchie46]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
We would like optional IO features

Describe the solution you'd like
Putting certain IO logic behind a feature gate and thereby reducing compilation size (including some dependencies)

[Dandandan]

From the thread on Slack:

what about separating json / csv from the arrow crate just like the parquet crate? IMO this is easier to manage and also easier to use/understand.
[...] I think features / conditional compilation introduce more complexity than a separate crate would in terms of organization, discoverability and testing.