[mlr] introduce Mlr namespace and rename types (#13053)

This commit introduces the `Mlr` namespace to encapsulate all
Multicast Listener Registration related types and logic, improving
overall code organization and readability.

The following primary renames were performed:
- `MlrManager` to `Mlr::Manager`
- `MlrState` to `Mlr::State`
- `MlrStatus` to `Mlr::Status`
- Constants like `kMlrSuccess` to `Mlr::kStatusSuccess`

Additionally, methods within the newly scoped `Mlr::Manager` class
have been simplified by removing redundant `Mlr` prefixes (e.g.,
`SendMlr()` is now `Send()`, `FinishMlr()` is now `Finish()`).

External modules and tests have been updated to reference the new
scoped names.

src/core/common/notifier.cpp

@@ -138,7 +138,7 @@ void Notifier::EmitEvents(void)
     Get<MeshCoP::BorderAgent::Admitter>().HandleNotifierEvents(events);
 #endif
 #if OPENTHREAD_CONFIG_MLR_ENABLE || (OPENTHREAD_FTD && OPENTHREAD_CONFIG_TMF_PROXY_MLR_ENABLE)
-    Get<MlrManager>().HandleNotifierEvents(events);
+    Get<Mlr::Manager>().HandleNotifierEvents(events);
 #endif
 #if OPENTHREAD_CONFIG_DUA_ENABLE || (OPENTHREAD_FTD && OPENTHREAD_CONFIG_TMF_PROXY_DUA_ENABLE)
     Get<DuaManager>().HandleNotifierEvents(events);