Add an Either type for Listeners

[qrnch-jan]

Is your feature request related to a problem? Please describe.
I want to be able to (easily) listen on either a UnixListener or TcpListener in the same function. Or, more precisely, I would like the freedom to nest other types as well, just as can be done with tokio-util's Either.

On the client side we have a function which parses the socket address string and detects if it's a local domain socket or not, and paired with a custom connection function we can easily allow the user to control whether to use local domain sockets or TCP simply by altering a configuration string. We want to be provide the same simplicity on the server end for the binding/listening.

Describe the solution you'd like
It would be nice to have an EitherListener (or equivalent) which works exactly like tokio-utik's Either, except that it works on the server end (i.e. .bind() and .accept()).

Describe alternatives you've considered
We have a hardwired solution, but the nice thing about Either is that it generic and supports nested types.

[Darksonn]

Adding this to tokio-util is fine with me.

[hi-rustin]

I am working on this.

[hi-rustin]

@Darksonn When you have time, could you help describe what you think the right API looks like?

[Darksonn]

Well there are probably several options. You could have an enum with the listener types hard-coded. You could also define a listener trait, which would have a poll_accept method, and have the enum's accept method work via that. If you use the trait, you could add the feature to the existing Either enum instead of adding a new one.

[Hodkinson]

Hi @hi-rustin how's it going with this one? I could have a look at it this week if you like.

[Darksonn]

Please note that this PR was recently posted: #4385

[Darksonn]

Fixed in #4385.