Support new JetStream API

[dan-j]

There's a new (well, 12 months old) SDK for interacting with JetStream, package github.com/nats-io/nats.go/jetstream. It makes things a bit easier for interacting with JetStream so we want to use it in https://github.com/knative-extensions/eventing-natss but there's a different implementation for the underlying message, jetstream.Msg instead of *nats.Msg.

It would be worth adding support to this SDK, but since our github.com/cloudevents/sdk-go/protocol/nats_jetstream/v2.Message implementation exports both the Message struct and the Msg field, it would technically be a breaking change:

sdk-go/protocol/nats_jetstream/v2/message.go

Lines 29 to 34 in 6408c01

	// Message implements binding.Message by wrapping an *nats.Msg.
	// This message *can* be read several times safely
	type Message struct {
	Msg *nats.Msg
	encoding binding.Encoding
	}

This new SDK also requires nats-server >= 2.9.0.

What are people's opinion on this? If interested, should we just add a new package supporting both and deprecate the old one in future? Any ideas on what this package should be?