Merge the relational and function metamodels

Having the function and relational models is no longer necessary. We can add the needed function metadata (including TEIIDDES-152 vararg support, null-on-null, etc.) through either extension properties or a metamodel change. This is in the vain of PostgreSQL which uses the create function syntax to add what would be procedures in other dbmses. Virtual functions (procedures marked as functions in a view layer) could also still specify transformation sql if pushdown is not required - rather than forcing the use of a java class.

We could deprecate the old function model support of the Teiid side then.