Add possibility for enumeration types to "extend" another enumeration type

[Alternative to ODATA-494]

We currently don't have a way for adding values to an enumeration type defined by a different schema. This would be helpful for defining custom vocabularies that extend base vocabularies.

However, defining inheritance for enumerations introduces substitutability that is likely to break clients by returning new enumeration values not anticipated by the client (for instance, in a switch statement).

An alternative to inheritance would be to introduce the ability for an enumeration type to definitionally extend an existing enumeration type. This is similar to what we do with entity containers, which can extend the definition of an existing container without introducing any substitutability.

We could even use the same "Extends" attribute, rather than "BaseType" which implies inheritance.