iot/zoap: Add a helper to find an observer by address

In some situations, for example, when the remote side sends a RESET
message indicating that it is no longer interested in observing a
resource, it is helpful to have a way to obtain the obverser
representation.

Change-Id: Ifbf627f9170be844fd525c557dda8cb722ac7aff
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>