feat: make validator buildable for outputter

[jkowalleck]

• add BY_SCHEMA_VERSION: Dict[SchemaVersion, Type[Xml|Json]]
• rework output.get_instance() to use dicts BY_SCHEMA_VERSION

make validators fetchable, if only an outputter was known

• add property output_format: OutputFormat to all outputters
• add property output_format: OutputFormat to all validators
• add validation.get_instance() analog to existing output.get_instance()

showcase the behaviour in the examples