Function check_unknown 

fn check_unknown(value: &Value, path: &str) -> Result<(), InvalidRecord>

Checks an unknown value.

unknown is not “anything”. atproto’s data model says the value is an open object, and the three container shapes — bytes, cid-link and blob — are excluded because each of them is a distinct node in the data model rather than a map, and a consumer that re-encoded one as a plain object would change what it is.