fix: return str placeholder in settings info if default value of sett…

…ing is a callable (#52)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Enhanced settings information display by providing clearer
representation of callable defaults in the settings.

- **Bug Fixes**
- Improved clarity and usability of the `get_settings_info` method
output.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

snakemake_interface_common/plugin_registry/plugin.py

@@ -89,6 +89,8 @@ def get_settings_info(self) -> List[Dict[str, Any]]:
 
             def fmt_default(thefield):
                 if thefield.default is not MISSING:
+                    if callable(thefield.default):
+                        return "<function>"
                     return thefield.default
                 else:
                     return None