Support projections in metadata schema SQL query

[machristie]

Currently the SQL query will return all matching DataProduct instances. So, for example, it only ever makes sense to issue a query that begins SELECT * FROM .... But it could be useful to return only a subset of metadata schema fields, both because it would be a smaller payload and because the client wouldn't need to retrieve the metadata schema field values from the 'metadata' field of each DataProduct.

It would be good to develop a compelling use case first though.