Add derives option. Add option to apply AuxMessageOptions based on a regex string

[oscar-broman]

This adds functionality which allows doing the following (among other things):

option (scalapb.options) = {
  aux_message_options: [
      {
          target_regex: "org.example\\..*"
          options: {
              derives: "io.circe.Codec.AsObject"
          }
      }
  ]
};

This will add the derives clause to all messages starting with org.example.

The two features are separate and the ability to use target_regex works for other settings also.

[oscar-broman]

@thesamet I've modified it such that the checks should succeed now, please re-run!