Provide hooks to interrogate metadata prior to full import

To support the Desinger we should offer the ability to interrogate metadata prior to full import.

Exploring metadata is effectively an entirely different mode of operation with respect to the current metadata processing logic on the Teiid side. Also partial metadata isn't something that would neatly be expressed through DDL - tables without columns, a list of schema names, etc.

Ways around that would be to expose source procedures for metadata interrogation:

getTableNames - which would probably give both the Teiid name and the name in source and consider the current translator metadata settings
getProcedureNames

And importer specific info such as for JDBC getTableTypes, getCatalogNames, getSchemaNames

I'd want to keep it fairly high level though. Getting column or key information I'd expect would be done through the normal full import.