[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.

tests/unit/test_lowpan.cpp

@@ -104,7 +104,7 @@ static void Init(void)
     otMeshLocalPrefix meshLocalPrefix = {{0xfd, 0x00, 0xca, 0xfe, 0xfa, 0xce, 0x12, 0x34}};
     OffsetRange       offsetRange;
 
-    sInstance->Get<Mle::MleRouter>().SetMeshLocalPrefix(static_cast<Ip6::NetworkPrefix &>(meshLocalPrefix));
+    sInstance->Get<Mle::Mle>().SetMeshLocalPrefix(static_cast<Ip6::NetworkPrefix &>(meshLocalPrefix));
 
     // Emulate global prefixes with contextes.
     uint8_t mockNetworkData[] = {