Support workspace trust

[rgrunber]

See redhat-developer/vscode-java#1926 for reference.

Current options/capabilities under consideration for restriction in an untrusted workspace :

• xml.java.home & xml.server.vmargs
  • We partly restrict these. It can be set in the workspace, but we detect it and prompt the user for xml.java.home and restrict javaagent for xml.server.vmargs
• xml.server.binary.{path,args,trustedHashes}
  • We already restrict these and can't be set by the workspace
• xml.validation.resolveExternalEntities
  • We don't restrict this at all, so we could do that, and certainly restrict it with workspace trust
• Downloading the XML server binary : should we restrict this entirely when untrusted ?
• xml.validation.enabled
  • Should this be set to false when a workspace is untrusted ?