Redo GraphQL enum fixing logic

[rafalp]

This PR removes existing utilities for fixing GraphQL enum handling and introduces two new ones:

• repair_schema_default_enum_values: repairs Python representation of default GraphQL enum values in schema.
• validate_schema_default_enum_values: validiates that GraphQL enum values in schema are valid.

Fixes #1136

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (010f551) 97.85% compared to head (6f1f4da) 97.85%.
Report is 11 commits behind head on master.

Files	Patch %	Lines
ariadne/enums_values_visitor.py	97.19%	5 Missing ⚠️
ariadne/enums.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1138    +/-   ##
========================================
  Coverage   97.85%   97.85%            
========================================
  Files         106      109     +3     
  Lines        7677     7865   +188     
========================================
+ Hits         7512     7696   +184     
- Misses        165      169     +4     

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