Add DNS configuration options to client builder

[mccordryan]

Hello!

I recently found myself in a situation where I needed to manually specify a DNS configuration (i.e. always use Cloudflare/Google/etc.) instead of using the system configuration.

I solved this fairly easily by implementing a new struct with the Resolve trait, and passing said struct in Client::builder::dns_resolver().

That said, I can't shake that this feels a little redundant, especially when reqwest is using hickory resolver (also used for my implementation) under the hood.

In my ideal world, I could pass a hickory_resolver::config::ResolverConfig directly to the client builder. Something like Client::builder::with_dns_config(SomeResolverConfig)

Not a massive issue by any means, but a smaller QoL change that would make things easier for my use case anyways. I'd be happy to open a PR, provided that you feel this would be an appropriate addition. I know the existing Resolve trait was added to be as generic as possible, so I understand if this is too niche.

[seanmonstar]

I've purposefully kept most dependencies from being part of the public API, so I can upgrade versions and not have it be breaking changes.

If there were common options that could be applied generally to any resolver built-in, we could add those. Unfortunately, getaddrinfo doesn't really allow any customization, other than your /etc/gai.conf file.