Effectful custom DNS resolution in blaze client

[ghost]

Make Http1Support.getAddress and BlazeClientBuilder.customDnsResolver: RequestKey => F[Either[Throwable, InetSocketAddress]]. The usage in makeClient is already effectful.

[rossabaker]

The customDnsResolver is going to be tricky to do in a bincompat fashion, but it seems like the right thing to do. Are you aiming for 1.0, or do you need this in 0.23?

[armanbilge]

It'll be slightly hacked but we can do this bincompatibly by leveraging type erasure.

Edit: nvmd, spoke too soon. It's exposed as a public val in the builder so that won't work.

[rossabaker]

I think we could deprecate the val and the current setter. If the setter is called, the function could be lifted.

[armanbilge]

This should be migrated I think, along with a few other issues that have been labelled since the schism.