josh-proxy: allow listening on unix sockets + systemd socket activation

[flokli]

I'd like to bind josh-proxy to a unix socket, or even use systemd socket activation, so I can lazily start josh-proxy on the first connection.

This seems to be well supported in the tokio-listener crate, which provides clap integration too:

  <LISTEN_ADDRESS>
          Socket address to listen for incoming connections.
          
          Various types of addresses are supported:
          
          * TCP socket address and port, like 127.0.0.1:8080 or [::]:80
          
          * UNIX socket path like /tmp/mysock or Linux abstract address like @abstract
          
          * Special keyword "inetd" for serving one connection from stdin/stdout
          
          * Special keyword "sd-listen" or "sd-listen-unix" to accept connections from file descriptor 3 (e.g. systemd socket activation)

This is a more general fix to a followup of #1285 I wanted to work on - rather than just adding support for IP to bind on, we'd gain support for all the other things you can bind on.

This would however mean we'd need to introduce another "listen address" cli arg (and deprecate the port one).

[christian-schilling]

I think there is no reason not to do this 👍🏻

[flokli]

I decided to refactor the CLI parsing to clap derive first: #1292

[flokli]

This turned out to be a bit more complex, because of the whole logic in josh to create a symlink to itself as a git update hook, and then curling the server. If we end up supporting different listen methods, it doesn't necessarily know its "external URL".

I need to think more about this, we could add a --external-url CLI flag that'd somehow propagate down into the update hook, but it feels all a bit ugly. Do you have any ideas about that?