Add JetStream NATS Client Extensions

[mtmk]

This PR introduces an interface-based Core API dependency for the JetStream API.

As a result of this change, several internal APIs will need to be made public. This will enable application developers to extend some of the lower-level features. This aligns with our overall mission to establish INatsClient as the default API level, providing the opportunity to introduce lower-level functions from our library through INatsConnection.

• INatsConnection: Opened functions mostly useful for creating custom subscribers which are used in JetStream consumer implementation.

• NatsSubBase: Reviewed and documented subscription base class to be inherited for custom subscriptions.

• INatsSubscriptionManager: Exposed the subscription manager as an interface, only having the single function opened used in current implementations. This could be opened up more as we see opportunities or demand.

• JetStream NatsClientExtensions: Convenience extension to create JetStream context.