Feature: Add FixedUriConnector decorator to use fixed, preconfigured URI instead (#117 by @clue)

This can be useful for consumers that do not support certain URIs, such as when you want to explicitly connect to a Unix domain socket (UDS) path instead of connecting to a default address assumed by an higher-level API:

$connector = new FixedUriConnector(
    'unix:///var/run/docker.sock',
    new UnixConnector($loop)
);

// destination will be ignored, actually connects to Unix domain socket
$promise = $connector->connect('localhost:80');