Querying for a specific type of DNS record is broken

You currently can't list DNS records of a specific type only, because the `ListDnsRecordsParams` takes a `DnsContent` enum as the parameter for filtering the type, but DnsContent not only contains the record type, but also the record content, which is why reqwest fails to build a query string from it. For the application I'm using this in, I've implemented the filtering locally *after* fetching the records like [this](https://git.jcg.re/jcgruenhage/cloudflare-ddns-service/src/branch/main/src/network.rs#L87-L93), but I'd be happy about a proper solution for this.