Generate message type enums #9

Currently, message types are hard-coded by publishers and consumers. This is error prone and needs alignment between sender and receiver. To improve this, an enum (or constants) per domain and version should be generated for all supported languages, like with the generated factories.

Examples for PHP:

enum ConnectFourV1Type: string
{
    case JoinGame = 'ConnectFour.JoinGame';

    case GetGamesByPlayer = 'ConnectFour.GetGamesByPlayer';
}

enum ConnectFourV2Type: string
{
    case JoinGame = 'ConnectFour.JoinGame.v2';

    case GetGamesByPlayer = 'ConnectFour.GetGamesByPlayer.v2';
}

As shown in the example, the version may be omitted in V1 because of backwards compatibility (but not necessary, since the app isn't running somewhere).