Fix: 293: use python represenation of enum values for default values in schema

[rafalp]

Fixes #293

This PR updates EnumType to scan schema for fields where Enum is used for value, and default value is defined.

TODO

• Handle fields args field(arg: Enum = DEFAULT)
• Handle inputs fields arg: Enum = DEFAULT
• Handle fields inputs args field(arg: {field: DEFAULT})
• Nested field inputs args field(arg: {child: {field: DEFAULT}})
• Write tests
• Cleanup the code

[rafalp]

Superseded by #533