this introduces a `Server::listen_channel` interface. this is like the
existing `listen_tcp` or `listen_unix` methods, but instead applies to
an in-memory channel of `(read, write)` tuples.

this is motivated by work like penumbra-zone/penumbra#3588. to summarize
the motivation, it would be nice if we had a way to run a tower-abci
service in cargo tests, without needing to bind to an actual port, or
create a unix-domain socket within a temporary directory.

pushing this, as experimentation continues. this really does seem like
a very promising avenue for abci testing!