Option to compile static curl with c-ares

It might be interesting to allow users to compile libcurl with c-ares as the DNS resolver. We might be able to just add [c-ares-sys](https://crates.io/crates/c-ares-sys) behind a feature flag and point the libcurl build process to the bundled c-ares.

I'm not sure what best practices are for using c-ares, but in theory c-ares yields better performance or responsiveness for certain workloads that require looking up lots of unique URLs (web crawling, for example), as well as enables a few extra DNS features.