Support project-specific JDK

[anthonyvdotbe]

Since I'm unable to reopen #224 I'm opening a new issue.

What was implemented is very limited in comparison to what Red Hat's extension provides: with Red Hat's extension, I just need to specify each JDK (JDK 23, JDK 21, JDK 17, ...) once in my user settings. And then for each project I open, Red Hat's extension automatically detects the best JDK by inspecting the Maven POM and uses that for that project.

With the current solution, I need to (1) specify the JDK for each workspace and (2) manually keep the JDK in sync with the Maven POM.
Also the current solution doesn't support specifying the JDK on a folder-level, so I can't open 3 projects, which all require a different JDK, in a single workspace.