Add RequestReplyMode for JetStream

[mtmk]

To use this feature it needs to be explicitly turned on to avoid potential issues with existing installations.
Set RequestReplyMode option to Direct to turn it on:

var opts = new NatsOpts { RequestReplyMode = NatsRequestReplyMode.Direct };
await using var connection = new NatsConnection(opts);

JS Publish:

Method	Mode	Mean	Gen0	Gen1	Gen2	Allocated
PublishAsync	SharedInbox	136,716.0 us	-	-	-	3617.91 KB
PublishAsync	Direct	123,555.9 us	-	-	-	2359.89 KB
PublishConcurrentlyAsync	SharedInbox	8,862.1 us	250.0000	125.0000	46.8750	2992.3 KB
PublishConcurrentlyAsync	Direct	11,191.3 us	250.0000	125.0000	46.8750	2997.1 KB

KV:

Method	Mode	Mean	Gen0	Gen1	Gen2	Allocated
TryGetAsync	SharedInbox	142.3 us	0.2441	-	-	4.26 KB
TryGetAsync	Direct	135.0 us	-	-	-	2.25 KB
GetAsync	SharedInbox	160.9 us	0.2441	-	-	5.51 KB
GetAsync	Direct	177.0 us	0.2441	-	-	3.45 KB
TryGetMultiAsync	SharedInbox	1,574.0 us	41.0156	9.7656	1.9531	536.02 KB
TryGetMultiAsync	Direct	1,585.5 us	23.4375	5.8594	-	322.94 KB
GetMultiAsync	SharedInbox	1,768.5 us	56.6406	9.7656	1.9531	743.52 KB
GetMultiAsync	Direct	1,760.2 us	39.0625	15.6250	-	536.43 KB

we have performance gains for all except PublishConcurrently which is ~20% slower. shouldn't be a show stopper since it's not widely used and the feature is opt-in.

[mtmk]

feature toggle needs to be explicitly turned on to avoid existing applications being affected.

[wallyqs]

LGTM