Allow @DefaultValue to apply to methods, in which case the method will produce a default value. Flexible signatures can be supported with the new annotated method system.

This would be merged after #598.

@DefaultValue("name")
public @NonNull ArgumentParseResult<YourValue> defaultValue(
  final @NonNull CommandContext<YourSenderType> context
) {
  // logic...
  return theValue;
}

then in the method, @DefaultValue(provider = "name")

We'd have to make the value of @DefaultValue default to "", which should be fine. We need a small class for housing the name-provider mappings.