Find a way to support PORTFILE (or equivalent) but for non-web apps

[dsyer]

In spring-projects-experimental/spring-boot-testjars#80 we have been discussing this. Testjars relies on the app it is running to be a Spring Boot webserver. It then switches on the WebServerPortFileWriter and watches the PORTFILE to read the port of the running application. This is neat, but it doesn't work if the app is not a web-server (MVC or webflux essentially). Maybe the PORTFILE could be generalized in Boot so that other listeners could be activated in a similar fashion? Or maybe there's a better way.

[philwebb]

We probably need to think about this after #47288 has been merged and we have a clearer idea of the types of application that listen on a port but don't use org.springframework.boot.web.server.WebServer. It seems like spring-projects-experimental/spring-boot-testjars#80 might even want us to go further than simple port information.

Perhaps somewhat related is org.springframework.boot.test.http.server.LocalTestWebServer which is now used to determine the web server URL to use for testing. That's an example of a class in the spring-boot-test module being implemented in the spring-boot-web-server which is a pattern we may be able to use here.