Add support for hickory-dns for http client dns resolution

[jlizen]

Today, smithy's generated clients use native Hyper dns handling, which in turn delegates to the OS dns resolver.

This is not ideal for a couple reasons:

• cached dns resolution still requires a fairly expensive OS call
• delegating to OS in general doesn't play as nicely with async due to needing the blocking threadpool

Hickory dns would be nicer because it is all in-process, including with baked-in caching. This allows for true async dns resolution and generally much lower footprint.