MODE-1965 Added explicit validation around mixin types not being able to inherit non-mixin types.

This exposed a number of problematic areas in some of our extensions, that had to be fixed:
- teiid & ddl sequencer - both were using CNDs which defined mixins that inherited non-mixin types
- CMIS connector - was registering (via `NodeTypeTemplate`) mixins that inherited from non-mixin types, but was really using them as primary types.