Ngstack 1017 schema decoration nullable required properties

[JakovKnezovicc]

Changes

• Added new config option: api_platform_extras.features.schema_decoration.nullable_required.
• Wired the option into PropertyMetadataFactoryDecorator via compiler pass.
• Updated required-property logic for output schemas to optionally include nullable typed properties.
• Documented the new parameter in README.md config example.