- define struct for retention policy with message age and label generation
- add methods to create, access, and modify retention policies
- implement default values, cloning, equality, and debug formatting
- 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
- introduce `Retention` struct to define data retention period and label generation flag
- add `MessageAge` enum to represent different retention time units (days, months, years)
- implement default values for retention policy (5 years, generate label)
