Support joins in metadata schema SQL query

[machristie]

Some use cases:

• inner join between two metadata schema would return only data products that belong to both schemas, and one could also filter by fields in either or both schemas
• join to the parent data product and then you can filter by the parent's metadata schema fields but return their children

Some thoughts:

• would be nice if the client doesn't need to know how to do the join but the MetadataSchemaQueryExecutor handles it. Clients could issue a NATURAL JOIN in the query like so

select * from my_schema NATURAL JOIN other_schema;

• maybe register a virtual parent_data_product table so clients can issue a query joining to the parent data product, again, without having to know the details of how to do the join:

select * from my_schema NATURAL JOIN parent_data_product;