[border-agent] move TXT data management to TxtData class (#12120)

Moves the management of MeshCoP service TXT data from the
`BorderAgent::Manager` class into the `TxtData` class.

This change improves separation of concerns by isolating all TXT
data-related logic, including vendor TXT data, change callbacks, and
notifier event handling, within the `TxtData` class. The
`BorderAgent::Manager` is simplified and its responsibilities are
more focused.

A new public method, `Refresh()`, is introduced on `TxtData` to
provide a clear API for other modules to signal that the MeshCoP
service TXT data needs to be re-evaluated and updated.

src/core/common/notifier.cpp

@@ -132,6 +132,7 @@ void Notifier::EmitEvents(void)
 #endif
 #if OPENTHREAD_CONFIG_BORDER_AGENT_ENABLE
     Get<MeshCoP::BorderAgent::Manager>().HandleNotifierEvents(events);
+    Get<MeshCoP::BorderAgent::TxtData>().HandleNotifierEvents(events);
 #endif
 #if OPENTHREAD_CONFIG_MLR_ENABLE || (OPENTHREAD_FTD && OPENTHREAD_CONFIG_TMF_PROXY_MLR_ENABLE)
     Get<MlrManager>().HandleNotifierEvents(events);