Persist observe: Support tracking observe information over server res…

…tarts

Add in optional callbacks for when observe subscriptions start and stop.

Add in optional callbacks for when the observe counter is updated (can be
ratelimited) for a resource as well as track when a resource is deleted.

Add is support for new functions

coap_persist_observe() for server to add in tracked observe information.

coap_resource_set_observe_num() to update observe counter from tracked
information.

coap_observe_track() to define the optional callback functions.

coap-server has -t option to track observe information (most of the code
updates).

UDP sessions supported, as well as OSCORE over UDP.

Documentation updated

examples/coap-client.c

@@ -782,7 +782,7 @@ get_oscore_conf(void) {
   /* Need a rw var to free off later and file_mem.s is a const */
   buf = read_file_mem(oscore_conf_file, &length);
   if (buf == NULL) {
-    fprintf(stderr, "OSCORE configuraton file error: %s\n", oscore_conf_file);
+    fprintf(stderr, "OSCORE configuration file error: %s\n", oscore_conf_file);
     return NULL;
   }
   file_mem.s = buf;
@@ -808,7 +808,7 @@ get_oscore_conf(void) {
                                      NULL, start_seq_num);
   coap_free(buf);
   if (oscore_conf == NULL) {
-    fprintf(stderr, "OSCORE configuraton file error: %s\n", oscore_conf_file);
+    fprintf(stderr, "OSCORE configuration file error: %s\n", oscore_conf_file);
     return NULL;
   }
   return oscore_conf;