core (launcher): new `wait_for_service` option: if `wait_for_service` is used in `[DEFAULT]`, the service at the specified host/port is waited for before starting the backend. This is useful in the situation where we need to be sure that something is started before running, e.g. in a container.

[DEFAULT]
xmpp_domain = server1.test
email_server = libervia-backend.test
email_port = 8025
email_admins_list = admin@server1.test

[component file-sharing]
http_upload_public_facing_url = http://localhost:7777
http_upload_connection_type = http
http_upload_port = 7777