[mle] refactor MleRouter and Mle classes into a single Mle class (#11411)

This commit refactors the `Mle` modules and combines the `MleRouter`
and `Mle` classes into a single `Mle` class which now handles both
FTD and MTD functionalities.

The `MleRouter` and `Mle` classes were originally intended as
sub-classes, where the base class `Mle` would provide MTD and common
behaviors, and `MleRouter` would implement FTD-specific behaviors.
However, over the years and as new features were implemented, these
two classes became more intertwined, and the `Mle` class began to
include many FTD-related functions and interactions with `MleRouter`
private variables and methods.

This commit simplifies the code by combining the two into a single
class. The previous `mle_router.cpp` file is also renamed to
`mle_ftd.cpp` to indicate that it implements FTD-specific MLE
behaviors.

src/core/thread/dua_manager.cpp

@@ -291,7 +291,7 @@ void DuaManager::UpdateCheckDelay(uint8_t aDelay)
 
 void DuaManager::HandleNotifierEvents(Events aEvents)
 {
-    Mle::MleRouter &mle = Get<Mle::MleRouter>();
+    Mle::Mle &mle = Get<Mle::Mle>();
 
 #if OPENTHREAD_CONFIG_DUA_ENABLE
     if (aEvents.Contains(kEventThreadNetdataChanged))
@@ -420,7 +420,7 @@ void DuaManager::UpdateTimeTickerRegistration(void)
 void DuaManager::PerformNextRegistration(void)
 {
     Error            error   = kErrorNone;
-    Mle::MleRouter  &mle     = Get<Mle::MleRouter>();
+    Mle::Mle        &mle     = Get<Mle::Mle>();
     Coap::Message   *message = nullptr;
     Tmf::MessageInfo messageInfo(GetInstance());
     Ip6::Address     dua;