add new kind of primitive.NsResolver

**FEATURE REQUEST**

When our team deploys the nameserver, the nameserver needs to use a domain name to access, so I need a resolver that can resolve DNS.

```
p, err := rocketmq.NewProducer(
producer.WithNsResolver(primitive.NewDNSResolver([]string{"ns1.example.com:9876"})),
producer.WithRetry(2),
)
```