[Jetstream] Allow TLS Client Authentication

[NickLarsenNZ]

Looking at the docs, and the configuration struct for Jetstream, it doesn't seem like TLS Client Authentication can be used.

components-contrib/pubsub/jetstream/metadata.go

Lines 24 to 36 in e639685

	type metadata struct {
	natsURL string
	jwt string
	seedKey string
	name string
	durableName string
	queueGroupName string
	startSequence uint64
	startTime time.Time
	deliverAll bool
	flowControl bool
	}

I wonder if it is a trivial addition?

[NickLarsenNZ]

Looks like an else if could be added here:

components-contrib/pubsub/jetstream/jetstream.go

Lines 50 to 57 in e639685

	// Set nats.UserJWT options when jwt and seed key is provided.
	if js.meta.jwt != "" && js.meta.seedKey != "" {
	opts = append(opts, nats.UserJWT(func() (string, error) {
	return js.meta.jwt, nil
	}, func(nonce []byte) ([]byte, error) {
	return sigHandler(js.meta.seedKey, nonce)
	}))
	}

And the NATS client has an example of connecting via TLS:

https://github.com/nats-io/nats.go#tls