Client formatting hooks

[gmac]

Reorganize formatting hooks (composition mergers, error formatters, etc) into a concern that can be added to various objects. All output formatting methodology is now centralized on one object, and allows Client to be that object.

Breaking changes

• Removes description_merger composer option. Override merge_descriptions client method.
• Removes deprecation_merger composer option. Override merge_deprecations client method.
• Removes default_value_merger composer option. Override merge_default_values client method.
• Removes directive_kwarg_merger composer option. Override merge_kwargs client method.
• Removes root_field_location_selector composer option. Use root_entrypoints composer option.
• Return from on_error hook no longer populates GraphQL error message. Override build_graphql_error client method to build the desired error shape.