NewTopicSubscription creates a new `TopicSubscription`.

TopicRegistration encapsulates the subscription and handlers.

TopicRoutes encapsulates the default route and multiple routing rules.

TopicRule represents a single routing rule.

TopicSubscription internally represents single topic subscription.

TopicRegistrar is a map of <pubsubname>-<topic> to `TopicRegistration` and acts as a lookup as the application is building up subscriptions with potentially multiple routes per topic.