Provide a way to start smtp server on random free port

The underlying event loop supports `port=0` for letting the operating system pick a random free port.
This is useful when using the smtp server in unit tests.
However, aiosmtp does not work with `port=0` because it tries to connect to this port in `_trigger_server`, not to the port actually chosen by the OS:
```
mock_mail_server.py:30: in start
s._controller.start()
venv-3.12\Lib\site-packages\aiosmtpd\controller.py:306: in start
self._trigger_server()
venv-3.12\Lib\site-packages\aiosmtpd\controller.py:501: in _trigger_server
InetMixin._trigger_server(self)
venv-3.12\Lib\site-packages\aiosmtpd\controller.py:444: in _trigger_server
s = stk.enter_context(create_connection((hostname, self.port), 1.0))
C:\Program Files\Python312\Lib\socket.py:852: in create_connection
E OSError: [WinError 10049] The requested address is not valid in its context
```

[portpicker](https://pypi.org/project/portpicker/) is a possible workaround, but requires complex setup (port server daemon) to avoid race conditions.
It would be better if aiosmtpd directly supported `port=0`. This would involve using `server.sockets[0].get_sock_name()` for finding the port actually used. There would also need to be some way for the code starting the server to wait until the socket is opened so that the port is known and can be passed to the code under test.

I've hacked together an external solution by deriving from `Controller`:
```python
class ControllerWithSupportForRandomPort(aiosmtpd.controller.Controller):
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
self._port_known = threading.Event()

def _trigger_server(self):
# The InetMixin base class expects to be able to connect to
# self.hostname/self.port, which are the parameters passed to __init__.
# If port==0 (as in our tests), this does not work as the server chooses a random
# free port. Instead use the port that was chosen by the operating system.
assert self.server is not None
actual_name = self.server.sockets[0].getsockname()
self.hostname = actual_name[0]
self.port = actual_name[1]
self._port_known.set()
return super()._trigger_server()

def wait_for_port(self):
# Allows the unit test to wait until `self.port` is set correctly
# before accessing that.
self._port_known.wait()
```