Allow serializing SM state

It is useful in some applications to get a snapshot of the stream
management state for storage outside the process, in order to recover
from crashes and other things.  The get_sm_state already present is not
suitable for this because it returns a live object tied in to the
current context and such, and containing much unneeded internal-api
data.

Introduce xmpp_sm_state_set_callback which is called every time the
state changes with a serialized state, and a dual xmpp_sm_state_restore
which takes in this serialized state and sets up a new sm_state based on
that.

The serialization is considered opaque from the PoV of the API, but is
based on CBOR to facilitate easy debugging.

src/auth.c

@@ -1232,6 +1232,7 @@ static void _sm_enable(xmpp_conn_t *conn)
     send_stanza(conn, enable, XMPP_QUEUE_SM_STROPHE);
     conn->sm_state->sm_sent_nr = 0;
     conn->sm_state->sm_enabled = 1;
+    trigger_sm_callback(conn);
 }
 
 static int
@@ -1486,6 +1487,8 @@ static int _handle_sm(xmpp_conn_t *const conn,
         name = NULL;
     }
 
+    trigger_sm_callback(conn);
+
 err_sm:
     if (!name) {
         char *err = "Couldn't convert stanza to text!";