- introduce `Retention` struct for managing message retention policies
- add `new` constructor with age and generate_label options
- implement getter methods for age and generate_label
- add `set_generate_label` to control label generation dynamically
- provide examples and usage documentation
- include comprehensive unit tests
📝 docs(retention): document retention policy configuration
- provide detailed documentation for `Retention` struct
- include usage examples for creating and configuring retention policies
- explain the purpose of the `age` and `generate_label` fields
- describe the behavior of `set_generate_label` method
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