[mle] new API to trigger search for better parent (#8018)

This commit adds `otThreadSearchForBetterParent()` to start the
process on a child to search for a better parent while staying
attached to its current parent. This commit also adds a related CLI
command `parent search`.

include/openthread/instance.h

@@ -53,7 +53,7 @@ extern "C" {
  * @note This number versions both OpenThread platform and user APIs.
  *
  */
-#define OPENTHREAD_API_VERSION (234)
+#define OPENTHREAD_API_VERSION (235)
 
 /**
  * @addtogroup api-instance

include/openthread/thread.h

@@ -855,6 +855,17 @@ otError otThreadGetParentAverageRssi(otInstance *aInstance, int8_t *aParentRssi)
  */
 otError otThreadGetParentLastRssi(otInstance *aInstance, int8_t *aLastRssi);
 
+/**
+ * Starts the process for child to search for a better parent while staying attached to its current parent.
+ *
+ * Must be used when device is attached as a child.
+ *
+ * @retval OT_ERROR_NONE           Successfully started the process to search for a better parent.
+ * @retval OT_ERROR_INVALID_STATE  Device role is not child.
+ *
+ */
+otError otThreadSearchForBetterParent(otInstance *aInstance);
+
 /**
  * Gets the IPv6 counters.
  *