Make `dandi validate` use BEP032 schema when encountering BEP032 dandiset

[yarikoptic]

previously we did it via BIDSVersion field specification but we could make it "dynamic" based on folders. I think "explicit" better than implicit, so we should use BIDSVersion

TODOs

• make nwb2bids embed version marker compatible with validator
• us download bep032 schema and validate against it