Expose: custom_open_socket

[ReinierMaas]

XRef: #1

Alternative implementation that makes it possible to use a custom open_socket function.

[ReinierMaas]

Note: This doesn't expose enough information because socket address is hidden in the implementation in the curl library... We need access to the underlying curl-sys handle...

[ReinierMaas]

I have made changes to curl-rust that allow us to perform blacklisting at the socket level. Currently awaiting the fork to push those changes for review.

I still have to move the moment that we allow setting the custom_open_socket because as I am trying out this interface I detected that we can't communicate back to the call-site of get_async.

[SanderHageman]

LGTM! the DefaultOpenSocketHandler looks a bit cursed but I can't immediately think of a better implementation