feature: get retention API for a stream

We need to implement a GET retention API for a given stream. Spec like

```
curl --location 'https://demo.parseable.io/api/v1/logstream/teststream1/retention' \
--header 'Authorization: Basic YWRtaW46YWRtaW4='
```

Returns the json

```
[
    {
        "description": "delete after 20 days",
        "action": "delete",
        "duration": "1d"
    }
]
```

