[spinel] expose RestoreProperties as a public method within RadioSpinel (#9501)

When running border routers, users have the option to perform RCP
firmware updates. After completing the update, users prefer not to
restart or reinitialize the master device but only want to restart and
restore the RCP device.

Restarting can be initiated by the user; for example, they can restart
the RCP device by triggering the reset pin. However, after this
restart is done, certain RCP information needs to be restored, such as
the panid and extended address.

Typically, the master device can restore the RCP by triggering
`RecoverFromRcpFailure`, but this behavior is not
user-controllable. So there's a need to implement a callable API for
RCP restoration.

If the `RestoreProperties` is made public, users can actively restore
the RCP by calling it and some other public functions.

src/lib/spinel/radio_spinel.hpp

@@ -908,6 +908,14 @@ public:
      */
     static otError SpinelStatusToOtError(spinel_status_t aStatus);
 
+#if OPENTHREAD_SPINEL_CONFIG_RCP_RESTORATION_MAX_COUNT > 0
+    /**
+     * Restore the properties of Radio Co-processor (RCP).
+     *
+     */
+    void RestoreProperties(void);
+#endif
+
 private:
     enum
     {
@@ -1016,9 +1024,6 @@ private:
     void HandleRcpTimeout(void);
     void RecoverFromRcpFailure(void);
 
-#if OPENTHREAD_SPINEL_CONFIG_RCP_RESTORATION_MAX_COUNT > 0
-    void RestoreProperties(void);
-#endif
     void UpdateParseErrorCount(otError aError)
     {
         mRadioSpinelMetrics.mSpinelParseErrorCount += (aError == OT_ERROR_PARSE) ? 1 : 0;