Allow remote shutdown through URL call

When this option is activated, Ubooquity server can be shut down by sending a POST request to the "/public-api/shutdown" endpoint.

The POST body must contain the secret API key. E.g (using your own key):

curl --request POST --url http://localhost:2202/public-api/shutdown --data kkc9eox2fwtkl68bt943ldcf7oijlw