[mle] add discovery request callback (#5131)

This commit allows notifying the application a discovery request is
received, which is useful if the application wants to be more active
when a joiner comes.

tests/scripts/expect/cli-scan-discover.exp

@@ -88,7 +88,16 @@ wait_for "" "Done"
 send "discover something_invalid\n"
 expect "Error 7: InvalidArgs"
 
+set spawn_id $spawn_1
+expect "version=2,joiner=0"
+
 set spawn_id $spawn_3
+send "joiner start 123456\n"
+set timeout 10
+expect "NotFound"
 dispose
 
+set spawn_id $spawn_1
+expect "version=2,joiner=1"
+
 dispose_nodes