Support core affinity for subscription threads

[levb]

See https://natsio.slack.com/archives/C01AWP40KGF/p1684269701738519

An options API would be a good approach. Internally there could be a struct for various fields, one of the fields could be a cpu_set_t like member, which would define the core or core(s) that the NATS thread could run upon. Something similar to the posix CPU_SET/CPU_ZERO would be needed to allow for cores to be added to it in a sensible way - wouldn't want people to having to deal with cores in terms of masks.

Here are some possible additions to the API: Types: natsCoreSet APIs:

natsOptions_ClearCoreSet(natsCoreSet**)
natsOptions_AddCore(natsCoreSet**, int coreid)
natsOptions_RemoveCore(natsCoreSet**, int coreid)
natsOptions_ApplyCoreSet(natsCoreSet*)

The above would then have to be called before the NATs thread is created which would be before any natsConnection_Subscribe call is made, a possible example would be like so:

{
   natsCoreSet core_set;
   natsConnection* conn = nullptr;
   natsSubscription *sub = nullptr;

   natsOptions_ClearCoreSet(core_set);

   // NATS thread shall only run on cores 1,3 and 5
   natsOptions_AddCore(core_set,1)
   natsOptions_AddCore(core_set,3)
   natsOptions_AddCore(core_set,5)

   natsOptions_ApplyCoreSet(core_set)

   natsConnection_ConnectTo(&conn, nats_uri);
   natsConnection_Subscribe(&sub, conn, subj, msg_handler, nullptr);
   .
   .
   .

   while (running)
   {
   }
}