Support passing `Enum` types directly to `make_executable_schema`

[rafalp]

Passing Enum type to make_executable_schema directly should be supported.

This will also need a research to see what should be done in conflict situations like:

• Enum with name was already defined in schema, but has different values.
• Enum with name wasn't defined in schema (create GraphQL type or raise error? this is mostly for future compat with Ariadne server lib.)

[rafalp]

Done in #32