Introduce ranch_transport:peercert/1 callback

Author: juise

src/ranch_transport.erl

@@ -64,6 +64,7 @@
 	-> {ok, {inet:ip_address(), inet:port_number()} | {local, binary()}} | {error, atom()}.
 -callback sockname(socket())
 	-> {ok, {inet:ip_address(), inet:port_number()} | {local, binary()}} | {error, atom()}.
+-callback peercert(socket()) -> {ok, public_key:der_encoded()} | {error, atom()}.
 -callback shutdown(socket(), read | write | read_write)
 	-> ok | {error, atom()}.
 -callback close(socket()) -> ok.