Make PortforwardingContainer be reusable

[TomDeBacker]

This relates to #6410.

Would it make sense to have PortforwardingContainer make use of the reuse functionality? Since it extends GenericContainer it is possible out-of-the-box. As-is a singleton-like pattern is used so that only one sshd container instance is created and updated when needed.

It would allow the PortforwardingContainer to be reused and still offer the same features as-is (from initial observations). The startup time of PortforwardingContainer is negligible but it might provide a case to make everything more uniform (no more calling static methods on Testcontainers.exposeHost().

Is there a specific philosophy behind the current implementation? My proposal could be implemented like:

PortForwardingContainer portForwardingContainer = new PortForwardingContainer();
portForwardingContainer.start();
portForwardingContainer.addExposedHostPort(server.getAddress().getPort());

There might implications I have not thought about, please share your considerations or thoughts. I have proof-of-concept I'm willing to share if there is some traction towards this. Is this something testcontainers-java would benefit from?