Consume LSP custom services in a custom XML extension

[angelozerr]

Take a sample with LSP4XML maven extension which should consume JDT LS extension to search artifactId, groupId, version list with completion.

Given this API:

public interface MavenSupport {

  @JsonRequest("maven/searctArtifact")
  List<ArtifactResult> searctArtifact(ArtifactParams params);

}

This search feature must be done in a JDT delegate command handler, that's why maven completion participant needs:

• access to the language client instance created by the XMLLanguageServerLaunch
• cast this language client to a custom interface MavenSupport to consume custom maven/searctArtifact request.
• when XMLLanguageServerLaunch is created it shoud initialize the launcher with the MavenSupport interface (how to do that?)

[angelozerr]

I close this issue since now we support custom command with workspace comm1nd.