Add enum namespace override to metamodel

[benkamphaus]

Users should be able to specify a namespace for enum idents that are valid for particular enum attributes. This is necessary to ensure that enums with an identical meaning that appear in different contexts are not duplicated unnecessarily, and that they can still be used to enable joins or graph traversal between these contexts when appropriate.

Currently, the CANDEL schema has several non-conforming enum types (pfs/os etc. reasons, variant feature types) and these require special case handling in the schema browser. Ideally, the schema browser could use inference on a :unify.enum/namespace attribute (or similar) in the metamodel to resolve this instead.

This requires:

• the addition of the flat/compiled unify metamodel attribute
• a way to specify this in the Unify schema definition edn file
• use of the compiled attribute in the schema dashboard (other repo)
• use of the compiled attribute in the infer-schema task.