Che 7 plugin: vscode-java

[tsmaeder]

Create a Che 7 plugin (following up on eclipse-theia/theia#3778) and add it to the plugin registry.

• create a base image for jdt.ls-based tooling
• make a PR against https://github.com/eclipse/che-plugin-registry with a che 7 plugin containing vscode-java

[svor]

• The Che7 yaml was prepared: https://github.com/eclipse/che-plugin-registry/blob/java/plugins/redhat.java/0.0.1/meta.yaml;
• As a base image we can use https://github.com/eclipse/che-theia/blob/master/dockerfiles/remote-plugin-runner-java8/Dockerfile

But we can't test it as a Che plugin because che-theia-remote-extension has old version of Theia (it should be fixed by eclipse-che/che-theia#42) and we need to update Theia image to the newer version (eclipse-che/che-theia#39)