Add support for Azure Service Bus sessions

[jonekdahl]

As discussed in #828, this is a PR to add session support for Azure Service Bus.

Adds a new config option EnableSessions for Azure Service Bus. Session support must be explicitly enabled. When disabled, CAP works like before.

When sessions are enabled:

• RequiresSession is set on auto-created subscriptions
• The SessionId property is set on the Service Bus Message
  • uses the value of the cap-session-id header, when present
  • otherwise falls back to using the message id (similar to the handling of cap-kafka-key)
• RegisterSessionHandler is used instead of RegisterMessageHandler when consuming events

There is also some brief documentation on the feature, how to enable and use it (in English, possibly needs translation?).

As we don't use CAP on the consumer side ourselves, this part is unfortunately not well tested.

Suggestions welcome!

[yang-xiaodong]

LGTM, Thanks