This changes allows initializing FileSystemProviders at image run time. Since that is a behavioral change, the functionality must be unlocked with --future-defaults=all or --future-defaults=run-time-initialized-jdk (see also #10531).

Note that with this turned on, no FileSystemProviders are allowed in the image heap. Since those are referenced from common JDK classes, like java.io.File or java.nio.Path, and these classes unfortunately often end up in static variables, we have special handling for those to allow them in the image heap. However, this is not the case for custom Path implementations. So the general advice is to ensure that Path and File objects are created at image run time.

It allows to do the following that was previously not possible:

public class Main {
    public static void main(String[] args) throws Exception {
        URI uri = URI.create("myfs:///");
        try (FileSystem fs = FileSystems.newFileSystem(uri, Map.of())) {
            Path path = fs.getPath("/test.txt");
            Files.write(path, "Hello from myfs!".getBytes(StandardCharsets.UTF_8));
            String content = Files.readString(path);
            System.out.println(content);
        }
    }
}

The example assumes that META-INF/services/java.nio.file.spi.FileSystemProvider contains a custom FileSystemProvider that registers the myfs scheme.

Fixes #5134