Support late binding of topic names

[gkinsman]

JustSaying currently has no support for deciding which topic to publish to at publish time. Target topics are determined on startup through configuration.

We want to support deciding the topic to be published to at publish time to support publishing to tenanted topics.

Possible API:
WithTopicCustomiser(Func<Message, string originalTopic, string resultTopic> customiser)

Problems:

• How do we support creating SnsTopicByName's at publish time. Currently they're created on startup to ensure that infrastructure exists before publishing.
  • Support creation of these at publish time
    • Was attempted in JustSayingStack, had potential for deadlocks
    • Used by LambdaWorker
  • Require user to specify possible topics at startup, and fail at publish time if not registered
• Extend support of infrastructureless mode to add templated ARN's for topic providers.
  • Assumes infra already exists, don't need to create infra on the fly