code: public CommandLine addMixin(String name, Object mixin) {
          getCommandSpec().addMixin(name, CommandSpec.forAnnotatedObject(mixin, factory));
          return this;
      }
docs: Adds the options and positional parameters in the specified mixin to this command.
  <p>The specified object may be a {@link CommandSpec CommandSpec} object, or it may be a user object with
  {@code @Option} and {@code @Parameters}-annotated fields, in which case picocli automatically
  constructs a {@code CommandSpec} from this user object.
  </p>
  @param name the name by which the mixin object may later be retrieved
  @param mixin an annotated user object or a {@link CommandSpec CommandSpec} object whose options and positional parameters to add to this command
  @return this CommandLine object, to allow method chaining
  @since 3.0