Add retention periods for identity and trait data

### Is your feature request related to a problem?

Currently, all identity and trait data is retained indefinitely. We should add logic to be able to define retention periods. This could be customer defined, but might also be dependent on the plan. For example, on the free plan, we could limit the retention period to a lower amount.

This process may well not be cost efficient still as we would need to perform additional reads and writes to dynamodb which cost more than the storage.

### Describe the solution you'd like.

Implement functionality to remove identities and traits older than the defined retention period. This will need to be implemented in Postgres, and DynamoDB.

### Describe alternatives you've considered

Do nothing, continue to increase size of data in dynamodb, and postgres for private cloud / self-hosted.

### Additional context

This issue cannot be solved in dynamodb at least, until we implement at least [this related issue](https://github.com/Flagsmith/flagsmith/issues/4973). This would allow us to determine which identities to remove.