public Set<String> getSupportedOptions() {
    SupportedOptions so = this.getClass().getAnnotation(SupportedOptions.class);
    if (so == null) {
      return ImmutableSet.of();
    } else {
      return ImmutableSet.copyOf(so.value());
    }
  }
Analogous to {@link Processor#getSupportedOptions()}, here to allow extensions to report their
own.

<p>By default, if the extension class is annotated with {@link SupportedOptions}, this will
return a set with the strings in the annotation. If the class is not so annotated, an empty set
is returned.

@return the set of options recognized by this extension or an empty set if none
@see SupportedOptions
value/src/main/java/com/google/auto/value/extension/AutoValueExtension.java