add redhat-java extension for VSCode

[rafaeltuelho]

It would be good to add the redhat-java extension to the code-server image.

Given users will be touching Java code during the refactoring labs, it's good to have the IDE fully configured for Java with code assistance, intellisense, etc.

According to the extension docs you VSCode (universal version) does not come with an embedded JRE for the Language Server. It requires JDK 17 or later in the path. So, this code-server image need to provide JDK 17 or later.