Derive `Eq` for `DnsContent`

Currently, comparisons between `DnsContent` instances require explicit matching of the enum variants.

```rust
let dns1: DnsContent = ...;
let dns2: DnsContent = ...;

let is_same_record: bool = match (dns1, dns2) {
(DnsContent::A { content: ip1 }, DnsContent::A { content: ip2 }) => ip1 == ip2,
// ...
_ => false
};
```

Deriving `Eq` (and therefore `PartialEq`), `DnsContent` instances can be compared using the `==` operator, providing a more ergonomic API to interact with DNS records.

```rust
let dns1: DnsContent = ...;
let dns2: DnsContent = ...;

let same_record: bool = dns1 == dns2;
```