transport: introduce `git_transport_smart_remote_connect_options`
6fc6eeb66c40310086c8f059cae41de69ad4c6da removed
`git_transport_smart_proxy_option`, and there was nothing added to
replace it. That made it hard for custom transports / smart
subtransports to know what remote connect options to use (e.g. proxy
options).
This change introduces `git_transport_smart_remote_connect_options` to
replace it.
proxy: add a payload field for the proxy options
I don't quite recall what we do in the other places where we use this,
but we should pass this payload to the callbacks.
