Add flag to only keep one record under a given path

[csuwildcat]

Imagine a use case like 'bio' as a protocol path, where you only want to allow one bio object. The desire is to be able to add some $flag property that tells the DWN to only ever keep 1 record and the latest version of that record to come in under a path. The other ask is to make RecordsRead support not requiring a recordId and reading back the first record under a filter, if the recordId is not provided.

[thehenrytsai]

I think this is a dup of #467, keeping that one since it has more context and input, closing this one.