This is a simple PR which moves the `GetListener` function to a
`DefaultGetListener` function, and changes `GetListener` to be a
variable which by default points to the `DefaultGetListener` function.
This allows people who may exist quasi-downstream of Gitea to create
alternate "GetListener" functions, with identical signatures, which
return different implementations of the `net.Listener` interface. This
approach is expressly intended to be non-invasive and have the least
possible impact on the gitea codebase. A previous version of this idea
was rejected before: https://github.com/go-gitea/gitea/issues/15544 but
because of issues like: https://github.com/go-gitea/gitea/issues/22335 I
**really** think that recommending people configure proxies by hand is
exactly the wrong way to do things(This is why there is a Tor Browser.).
This tiny change lets me put proper hidden service configuration into
single `i2p.go` file which lives in `modules/graceful/` and which never
has to be checked in to your codebase or affect your dependencies or
bloat your project in any way, it can live on a branch in my fork and
I'll fast-forward every release and never the twain shall meet.

The main use-case for this is to listen on Peer-to-Peer networks and
Hidden Services directly without error-prone and cumbersome
port-forwarding configuration. For instance, I might implement an
"I2PGetListener" as follows:

```Go
// adapted from i2p.go which is unchecked-in in my modules/graceful/ directory
import "github.com/eyedeekay/onramp"

var garlic = &onramp.Garlic{}

func I2PGetListener(network, address string) (net.Listener, error) {
	// Add a deferral to say that we've tried to grab a listener
	defer GetManager().InformCleanup()
	switch network {
	case "tcp", "tcp4", "tcp6", "i2p", "i2pt":
		return garlic.Listen()
	case "unix", "unixpacket":
// I2P isn't really a replacement for the stuff you use Unix sockets for and it's also not an anonymity risk, so treat them normally
		unixAddr, err := net.ResolveUnixAddr(network, address)
		if err != nil {
			return nil, err
		}
		return GetListenerUnix(network, unixAddr)
	default:
		return nil, net.UnknownNetworkError(network)
	}
}
```

I could then substitute that GetListener function and be 50% of the way
to having a fully-functioning gitea-over-hidden-services instance
without any additional configuration(The other 50% doesn't require any
code-changes on gitea's part).

There are 2 advantages here, one being convenience, first this turns
hidden services into a zero-configuration option for self-hosting gitea,
and second safety, these Go libraries are passing around
hidden-service-only versions of the net.Addr struct, they're using
hidden-service-only versions of the sockets, which are both expressly
designed to never require access to any information outside the hidden
service network, manipulating the application so it reveals information
about the host becomes much more difficult, and some attacks become
nearly impossible. It also opens up TLS-over-Hidden Services support
which is niche right now, of course, but in a future where gitea
instances federate if hidden services want to be part of the federation
they're probably going to need TLS certificates. They don't need to be
painful to set up.

This doesn't fix an open issue, but it might affect:
- https://github.com/go-gitea/gitea/issues/22335 - my `i2p.go` file
actually has a mod that fixes this but it requires adding a handful of
new dependencies to gitea and isn't compatible with the normal way you
guys recommend using a proxy so I don't think it's ready to send to you
as a PR, but if I can find a non-invasive way to fix it I will.
 - https://github.com/go-gitea/gitea/issues/18240

I hereby agree to the Code of Conduct published here:
8b89563bf1/CODE_OF_CONDUCT.md
I have read and understood the recommendations published here:
8b89563bf1/CONTRIBUTING.md

Thank you for your consideration.

---------

Co-authored-by: eyedeekay <idk@mulder>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>