res_respoke: Support dynamic endpoint creation

Using ARI, endpoints managed by Asterisk's sorcery "ORM" can be
dynamically created/destroyed. Unfortunately, a bug in res_respoke kept
this functionality from working; in fact, attempting to create an
endpoint backed by a realtime or AstDB wizard would crash.

This crash occurred due to an infinite recursive call in
respoke_endpoint_alloc. Objects in Sorcery that are managed by certain
wizards are explicitly immutable, which necessitates that all Sorcery
objects be treated as immutable. res_respoke was storing a pointer to
the endpoint registration state on the endpoint Sorcery object. In order
to determine if the state already existed, a call to Sorcery's
'retrieve_by_id' function was made; in the AstDB and other wizards, this
creates a new instance of the endpoint object, which would attempt to
retrieve the endpoint object, etc.

The solution is to not store mutable state on the immutable object. A
global ao2 container now holds the endpoint states. This allows
endpoints created via Sorcery to query that container to see if they
have any state on object creation/deletion. In order to manage this
process, Sorcery observers have been added for object type registration
and for object deletion.

The result of this patch is that ARI PUT/DELETE commands now work with
res_respoke, and endpoints can be created on the fly dynamically.

res/res_respoke/respoke_endpoint.c

@@ -786,6 +786,7 @@ static const struct ast_sorcery_instance_observer observer_callbacks = {
 static void endpoint_deleted_observer(const void *obj)
 {
 	const struct respoke_endpoint *endpoint = obj;
+	struct respoke_endpoint_state *state;
 	struct ao2_container *states;
 
 	states = ao2_global_obj_ref(endpoint_states);