Axum 0.8 Listener impl for VsockListener

[kanpov]

Skimming over axum 0.8 while migrating my projects to it, I noticed serve is now properly generic over a listener type and thus not limited to TCP, but this support is done via a trait in the axum crate, meaning orphan rule happiness ensues.

Here is the trait in question:

Logically, there should be an axum/axum08 feature that adds the relevant impl.

[kanpov]

My attempt at implementing this is in #61.

[kanpov]

I'll make a small demo bin-crate that actually tests the current impl and then close this, to make sure everything works.