Add relational model extension framework (MED)

[blafond]

Our Komodo relational metamodel will need to support model and model object extensions.

In DDL, these extensions are basically OPTIONS(.......) clause properties.

Need to look at Teiid Designer's MED framework, maybe create a CND for our MED structure. Designer has a MED registry view and MED manager that can pull server-defined MED information, which could be server-version-specific.

Probably need to look at migrating/importing current Designer MED's into Komodo.