DNS: Support for additional record types

It seems like this crate currently only supports the most common DNS record types:

```rust
// in cloudflare/src/endpoints/dns.rs
pub enum DnsContent {
A { content: Ipv4Addr },
AAAA { content: Ipv6Addr },
CNAME { content: String },
NS { content: String },
MX { content: String, priority: u16 },
TXT { content: String },
}
```

However, the [Cloudflare API](https://developers.cloudflare.com/api/operations/dns-records-for-a-zone-list-dns-records) lists several additional record types that are currently not implemented, including `CAA`, `CERT`, `DNSKEY` and more.

From what I can tell, adding any of these records to a zone causes the `ListDnsRecords` query to fail with an `unknown variant` error during deserialization, effectively making the `ListDnsRecords` query unavailable for such a zone (example below).
I'm running into this issue on my own zones, since I have a `CAA` record set on them.

I have a [fork](https://github.com/spacebird-dev/cloudflare-rs/commit/8eb18ca21e7f76ad93288b5d7a3ec32e2b9002ce) that fixes this for `CAA` records, but I'd be interested in seeing support for these record types in this main crate.
Adding simple enum variants for these records with the `content` field should be sufficient to allow the `ListDnsRecords` request to succeed (the API also returns a structured `data` field for some record types, but I don't think this is necessary for basic support).

If support for these record types is desired, I'd be more than happy to make the required changes and open a PR!

---

Steps to reproduce:

1. Create a zone and create a CAA record through the Web Interface (such as `0 issue letsencrypt.org`).
2. Attempt to list all records in that zone using the `cloudflare-examples` utility like so:

```
cargo run -p cloudflare-examples -- --auth-token dns
```

3. The client will return the following error:

```
Error: error decoding response body: unknown variant `CAA`, expected one of `A`, `AAAA`, `CNAME`, `NS`, `MX`, `TXT`, `SRV` at line 1 column 8705
```