externalize Initializr-to-template option mapping

[ccheetham]

Some Initializr and template options are functionally identical but use different names. An example is adding a RabbitMQ connector; the Initializr option is amqp while the template option is rabbitmq.

The current implementation maintains the mapping in code.

The InitializrApi should provide the ability to use an externally configured mapping, e.g. in appsettings.json so that mappings can be maintained without having to rebuild the server.