fix(graphcache): Replace default optional/required directive implementation

[kitten]

Summary

This replaces the default implementations for @_required and @_optional. These directives are now handled by checking for field._directives.required and field._directives.optional directly inside the readSelection logic.

This should enhance @_optional to also work on fields with selection sets properly.

Set of changes

• Remove defaultDirectives from implementation
• Add handling of directives.optional and directives.required directly to readSelection