INT-611: Introduce extendable messaging service

[reinkrul]

This introduces a messaging service, which lays the groundwork for reliable event and FHIR subscription notification handling. It can be extended with more topics/queues to be published on and message receivers. Additional changes:

• Mostly reimplements the existing Azure Service Bus integration for delivery of the enrolment bundle (renamed to "task filler/task accepted bundle" since Task workflows aren't just for enrolments).
• Removes the "debug" (write messages to disk) feature of the ServiceBus integration, since it was used during development, but not afterwards. Use unit tests, or the HTTP messaging endpoint instead.
• Removes the "ping" (sends a text message "ping" to a topic on startup) feature of the ServiceBus integration, since it was used during development, but will cause increased ops-load on test/acc/production environments (since it's a message that most probably can't be processed, it'll end up in the DLQ).
• Moved the delivery of enrolment bundles over HTTP (a.k.a. "demo client") to a separate HTTP message broker implementation.
• Moved configuration of message broker from CPC to ORCA root, since it'll also be used by CPS.
• Removed the Azure Service Bus setup in the local deployment, introduced an integration test instead. Reason: it starts slowly and adds additional overhead to the (already) heavy-weight local-dev deployment. We'll support an in-memory broker very soon, which is to be preferred for local-dev deployment.
• Improved documentation