BugFix: Enum support in compound keys

Fixes issue #23

When using Enum Field as a key in containing a compound statement, validation fails and doesn't work (raises AssertionError: Invalid compound key definition for type). But the same works if there is no compound key, only enum key.

Changes:

fix(Compound Key): Add validation for EnumType, fixes Invalid compound key definition for EnumType
feat(Tests): Add test for compound key with Enum types
refactor(Compound Key): cleaner logic

graphql-python / graphene-federation

BugFix: Enum support in compound keys #24