Making serde an optional dependency

[LucaCappelletti94]

Hi,

I see that there existed a PR (#175) attempting to add this feature, and several comments are in that PR requesting this feature. Nevertheless, the PR was rather old and the author closed it down.

I will attempt to make a new PR adding support for this feature, as I need to use this CSV parser inside apache/datafusion-sqlparser-rs#2077

Best,
Luca