Feature Request: macOS socket listener support

[dotsam]

I believe that ssh-agent-lib should expose the listening capabilities of service-binding, which supports socket activation with both systemd and macOS' launchd: https://github.com/wiktor-k/service-binding?tab=readme-ov-file#launchd-socket-activation

I'd love to see this supported as a feature, as this allows for use of launchd's SecureSocketWithKey abilities, which exports a secure path as an env variable. This is how macOS's built-in SSH agent works.

In the past, I've used this functionality with a small wrapped for the gpg-agent to have it export SSH_AUTH_SOCK in a way that it's available to all GUI apps by default as well: https://gist.github.com/dotsam/30d179ef9e005a149b118b72a13e32e7

I believe this method relies on service ordering however, as two SSH_AUTH_SOCKSs get set, and the last one is the one that's used:

$ launchctl print gui/501
gui/501 = {
	type = login
	handle = 100020
	active count = 457
	service count = 456
	active service count = 225
	creator = loginwindow[583]
	creator euid = 0
	session = Aqua
	endpoint destination = com.apple.xpc.launchd.domain.user.501
	auxiliary bootstrapper = com.apple.xpc.otherbsd (complete)
	security context = {
		uid = 501
		asid = 100020
	}

	bringup time = 978 ms
	death port = 0x14003

	environment = {
		SSH_AUTH_SOCK => /private/tmp/com.apple.launchd.ArRQ2YQzkd/ssh
		SSH_AUTH_SOCK => /private/tmp/com.apple.launchd.JikDe3G8qT/Listeners
		PATH => /opt/homebrew/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
	}

	services = {
		...

This might also complicate #29, depending on how things are implemented