add endpoints for concept-graph status checking

[fmatthies]

There are already endpoints implemented for checking the status of a process (in the concept-graphs-api as well as in the ConceptGraphsRepository, but not in the top-api:

  public enum API_STATUS {
    SELF("/status"),
    DOCUMENT_SERVER("/status/document-server");

    private String endpoint;

    API_STATUS(String endpoint) {
      this.setEndpoint(endpoint);
    }

    public String getEndpoint() {
      return endpoint;
    }

    public void setEndpoint(String endpoint) {
      this.endpoint = endpoint;
    }
  }

I imagined that to be the method to periodically check from the frontend whether the process started is already finished/at what point in the process it is.