support for custom feature encoding/decoding
@lhoestq  The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7284). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.
Fix for https://github.com/huggingface/datasets/issues/7220 as suggested in discussion, in preference to #7221 

(only concern would be on effect on type checking with custom feature types that aren't covered by FeatureType?)