Add VS code extensions preferences in workspace configuration

Description

For example it should be possible to override the default JDT LS vmargs to match the sidecar container memory settings. Today that’s not possible and we have 2 workarounds that are not really satisfactory:

• use a sidecar with > 1.5GB (may be too much for che.openshift.io)
• start the workspace and edit the JTD LS vmargs from Theia settings (but the UX is awful)

That same problem will exist for any VS Code plugin that has some defaults that may not work well when they run in a container.

Here is a proposal to include that in a devfile and workspace.config:

  - name: java support
    type: chePlugin
    id: 'org.eclipse.che.vscode-redhat.java:0.38.0'
    preferences:
       java.jdt.ls.vmargs: '-noverify -Xmx1G -XX:+UseG1GC -XX:+UseStringDeduplication'
       java.home: '/home/user/jdk11'

devfile

and in json config

"attributes": {
  "plugin.org.eclipse.che.vscode-redhat.java.java.jdt.ls.vmargs": "-noverify -Xmx1G -XX:+UseG1GC -XX:+UseStringDeduplication",
  "plugin.org.eclipse.che.vscode-redhat.java.java.home": "/home/user/jdk11"
...
}

workspace config

[azatsarynnyy]

Also, that Theia extension should take into account that not all of the Che Workspace attributes are the settings for Theia. The extension should be able to distinguish it. Such attributes can be prefixed with preference, e.g.:

"attributes": {
  "plugin.org.eclipse.che.vscode-redhat.java.preference.java.jdt.ls.vmargs": "-noverify -Xmx1G -XX:+UseG1GC -XX:+UseStringDeduplication",
  "plugin.org.eclipse.che.vscode-redhat.java.preference.java.home": "/home/user/jdk11"
...
}

[benoitf]

@tsmaeder : well I would say it’s because workpace config is only allowing attributes in the current model.