The file sink configures Boundary to send events to a file.

```hcl
sink {
    name = "obs-sink"
    description = "Observations sent to a file"
    event_types = ["observation"]
    format = "cloudevents-json"
    file {
      path = "/var/log/boundary"
      file_name = "events.ndjson"
    }
  }
```

The `sink` stanza may be specified more than once to make Boundary send events
to multiple sinks; however, each file sink must have a unique `path` +
`file_name`.
