Functionality to construct partial record

[ericphanson]

If we have some subset of fields of a record whose construction/validation transformations do not depend on other fields, it would be nice to be able to construct the partial record (even as a NamedTuple or such) with this validation applied. For example, let's say the schema of interest is an Onda.SignalV2, and I only have the recording and file_format fields. It would be nice to be able to call the same code to run the constructor (e.g. call UUID on the recording field) for those fields, giving back a NamedTuple that represents the partial record.

This is handy for efficient interop with GraphQL, which allows only retrieving the fields you need.