Add direct request-reply

[mtmk]

With this PR we make the request-reply more efficient by skipping inbox queues, hopefully eliminating of the timeout issues some applications have been experiencing.

Opts.RequestReplyMode Feature flag

Using this flag applications can opt-in to this feature minimizing the risk of experiencing issues where somehow they rely on the existing behavior.

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

Benchmark

RequestReplyBench:

Method	Mean	Error	StdDev	Ratio	Gen0	Gen1	Allocated	Alloc Ratio
RequestReplyAsync	160.9 us	26.41 us	1.45 us	1.00	0.2441	0.2441	4.57 KB	1.00
RequestReplyDirectAsync	147.4 us	19.57 us	1.07 us	0.92	-	-	2.61 KB	0.57

[caleblloyd]

Nice improvement! LGTM