Add ClusterTraffic field to NatsAccount

## Description

The Go `Account` struct has a `ClusterTraffic` field that is missing from the .NET `NatsAccount` model.

Go definition (in `account_claims.go`):
```go
type Account struct {
    // ...
    ClusterTraffic *ClusterTraffic `json:"cluster_traffic,omitempty"`
    // ...
}
```

This should be added to `NatsAccount` for full parity with the Go implementation.