Add config option to have RuntimeRunner save generated classes to disk

[starksm64]

It came up on zulip chat that several were having to augment the RuntimeRunner to output the classes it writes to disk for inspection. @gsmet had posted this code:

final Path wiringClassesDirectory = Files.createTempDirectory("wiring-classes-");
            ClassOutput classOutput = new ClassOutput() {
                @Override
                public void writeClass(boolean applicationClass, String className, byte[] data) throws IOException {
                    String location = className.replace('.', '/');
                    final Path p = wiringClassesDirectory.resolve(location + ".class");
                    Files.createDirectories(p.getParent());
                    try (OutputStream out = Files.newOutputStream(p)) {
                        out.write(data);
                    }
                }

                @Override
                public void writeResource(String name, byte[] data) throws IOException {
                    final Path p = wiringClassesDirectory.resolve(name);
                    Files.createDirectories(p.getParent());
                    try (OutputStream out = Files.newOutputStream(p)) {
                        out.write(data);
                    }
                }
            };

We should have a config option to enable this behavior.

[gsmet]

Apparently, it was there all along and we missed it:
https://github.com/jbossas/protean-shamrock/blob/master/core/deployment/src/main/java/io/quarkus/runner/RuntimeClassLoader.java#L68

mvn clean install -Dio.quarkus.DEBUG_GENERATED_CLASSES_DIR=./target/my-generated-classes

[mkouba]

Should we add a note to the Extension Authors Guide?

[starksm64]

I did create a PR to add this to the extension author guide.