[style] harmonize feature check and header includes (#6399)

This commit harmonizes how the feature config checks are done within
OT core modules. In header `.hpp` files, any related `#if` config
check is done immediately after the `"openthread-core-config.h"` is
included. This way the rest of definitions are skipped over if the
feature is not being used. In `cpp` source files the `#if` check is
done immediately after including the related header file.

src/core/api/thread_api.cpp

@@ -33,6 +33,8 @@
 
 #include "openthread-core-config.h"
 
+#if OPENTHREAD_FTD || OPENTHREAD_MTD
+
 #include <openthread/thread.h>
 
 #include "common/debug.hpp"
@@ -42,7 +44,6 @@
 
 using namespace ot;
 
-#if OPENTHREAD_FTD || OPENTHREAD_MTD
 uint32_t otThreadGetChildTimeout(otInstance *aInstance)
 {
     Instance &instance = *static_cast<Instance *>(aInstance);
@@ -515,4 +516,5 @@ void otThreadRegisterParentResponseCallback(otInstance *                   aInst
 
     instance.Get<Mle::MleRouter>().RegisterParentResponseStatsCallback(aCallback, aContext);
 }
+
 #endif // OPENTHREAD_FTD || OPENTHREAD_MTD