feat: write listen files with actual address

[zepatrik]

It is now possible to set a write_listen_file URL in the config for each port. The server will write the actual listen address to that file, so it is now possible to easily use port 0.

[zepatrik]

I would also like to do this for all other services, so we can use this as a test playground for the feature.

[zepatrik]

No more racy freeport 🎉

[hperl]

Looks very nice! I just have some minor remarks ;)

[hperl]

Could we just return a struct with all the values here instead of a closure?

[zepatrik]

No not really. The test has to do these steps in this order:

1. create the registry
2. call UseDynamicPorts to set the config values
3. start the server(s); potentially only some and not all
4. use the closure to retrieve the actual address the server is listening on

What we do with this change is let the OS choose a free port when the server starts listening (using port 0). We can only get the actual port number after the server was started, therefore this step comes later.
The problem with freeport is that their approach is to open a socket on port 0, get the actual port number, and then close it again. This results in racy behavior where a parallel test gets the same port assigned, because the socket is closed and then reopened.