Models implements serializable interface to support native image(GraalVM)

[nilchina]

Hi guys! When building a native image, the classes under the models package need to be registered in the form of serialization, so they need to implement the serialization interface.

[nilchina]

@configuration
@ImportRuntimeHints(MyRuntimeHints.class)
public class MyRuntimeHints implements RuntimeHintsRegistrar {
@OverRide
public void registerHints(RuntimeHints hints, ClassLoader classLoader) {
hints.serialization().registerType(org.gitlab4j.api.models.Project.class);
hints.serialization().registerType(org.gitlab4j.api.models.Namespace.class);
//......
}
}

[jmini]

This is fine for me. I think this PR could target main since I am merging all the changes on main also to 6.x (see #926 (comment))

We have a test class TestGitLabApiBeans to test all the models to JSON serialization and deserialization.
I make sure it stays up to date with each PR.

Probably we could add somewhere a test to make sure each class and its sub-class are implementing Serializable. Otherwise we can often miss one case.

[jmini]

I have made some changes to this PR:

• to target the main branch.

git rebase --onto origin/main origin/6.x

• to use 1L as serialVersionUID
• to add a test in JsonUtils to make sure we catch all the classes that we are converting to JSON (in the tests)

---

@nilchina please review.

---

@nilchina Be aware that your local branch 6.x has diverged with the one on the GitHub.
If you need to do one more change on this branch:
I recommend to use git fetch and git reset --hard origin/6.x before doing any local modifications instead of git pull.