Password support for airplay

[chapterjason]

Is your feature request related to a problem? Please describe.
I'm currently working on a sound system which wraps the snapcast services and set configuration and restart the services.
Adding authentication for airplay is actually not possible, cause the query is not applied if I use this stream: airplay:///shairport-sync?name=Airplay&devicename=Node-Test&password=admin

Describe the solution you'd like
shairport-sync supports setting password via argument .

https://github.com/mikebrady/shairport-sync/blob/272b3db5ffdc7be6d21216c0b602fe02f0cdf3b6/man/shairport-sync.7.xml#L50

Copy the password from the query like the devicename would be a solution.

snapcast/server/streamreader/airplay_stream.cpp

Line 55 in 9fd61ee

params_wo_port_ = "\"--name=" + devicename + "\" --output=stdout --use-stderr --get-coverart";

Describe alternatives you've considered
Already thought about change the airplay default configuration, but I think it is easier to add support for the argument.

Additional context

[badaix]

Fixed ae16e74