[cli] new command to enable/disable discovery request callback (#8579)

This commit add `discover reqcallback {enable/disable}` CLI command
which registers callback to output whenever device gets a "MLE
Discovery Request". This command allows the callback to be explicitly
set when needed and avoid having it be enabled by default (since the
extra output from this callback can be treated as unexpected result by
tests that parse the CLI output).

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

@@ -57,6 +57,8 @@ expect "channel"
 expect -re {(\d+)}
 set channel $expect_out(1,string)
 expect_line "Done"
+send "discover reqcallback enable\n"
+expect_line "Done"
 
 switch_node 3
 send "scan $channel\n"