Introduce a new callback that fires when the remote is ready to connect.

Since libssh2 doesn't read host configuration from the config file,
this callback can be used to hand over URL resolving to the client
without touching the SSH implementation itself.

These are small pieces of data, so there is no advantage to allocating
them separately. Include the two ids inline in the struct we use to
check that the expected and actual ids match.

Instead of having it set in a different place from every other callback,
put it the main structure. This removes some state from the remote and
makes it behave more like clone, where the constructors are passed via
the options.

It can be useful for the caller to know which update commands will be
sent to the server before the packfile is pushed up. git does this via
the pre-push hook.

We don't have hooks, but as it adds introspection into what is
happening, we can add a callback which performs the same function.

We have the step-by-step method in the initialization function as we
want to remove references based on the list of references which are
already there, and we can use the convenience function for testing the
main push.

This function, similar in style to git_remote_fetch(), performs all the
steps required for a push, with a similar interface.

The remote callbacks struct has learnt about the push callbacks, letting
us set the callbacks a single time instead of setting some in the remote
and some in the push operation.

If the certificate validation fails (or always in the case of ssh),
let the user decide whether to allow the connection.

The data structure passed to the user is the native certificate
information from the underlying implementation, namely OpenSSL or
WinHTTP.