[mle] add config and test for discovery request callback (#12284)

Introduces `OPENTHREAD_CONFIG_MLE_DISCOVERY_SCAN_REQUEST_CALLBACK_ENABLE`
to conditionally compile the MLE Discovery Request callback feature.
Disabling this feature allows for code size reduction on builds where
it is not needed.

A new Nexus test (`test_discover_scan.cpp`) is added to directly
validate the `otThreadSetDiscoveryRequestCallback()` API behavior.

This new test replaces a now-removed CLI-based test which used
 (`discover reqcallback`). This CLI command was originally added for
testing purposes. The CLI command is not helpful as an async event
would produce unsolicited output in the CLI. The new direct C++ test
is a cleaner approach.

src/core/thread/mle_ftd.cpp

@@ -2749,9 +2749,10 @@ void Mle::HandleDiscoveryRequest(RxInfo &aRxInfo)
 
     if (parsedDiscoveryRequestTlv)
     {
+#if OPENTHREAD_CONFIG_MLE_DISCOVERY_SCAN_REQUEST_CALLBACK_ENABLE
         if (mDiscoveryRequestCallback.IsSet())
         {
-            otThreadDiscoveryRequestInfo info;
+            DiscoveryRequestInfo info;
 
             AsCoreType(&info.mExtAddress).SetFromIid(aRxInfo.mMessageInfo.GetPeerAddr().GetIid());
             info.mVersion  = discoveryRequestTlvValue.GetVersion();
@@ -2759,6 +2760,7 @@ void Mle::HandleDiscoveryRequest(RxInfo &aRxInfo)
 
             mDiscoveryRequestCallback.Invoke(&info);
         }
+#endif
 
         if (discoveryRequestTlvValue.GetJoinerFlag())
         {