[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`.

src/core/api/thread_api.cpp

@@ -377,6 +377,11 @@ exit:
     return error;
 }
 
+otError otThreadSearchForBetterParent(otInstance *aInstance)
+{
+    return AsCoreType(aInstance).Get<Mle::Mle>().SearchForBetterParent();
+}
+
 otError otThreadSetEnabled(otInstance *aInstance, bool aEnabled)
 {
     Error error = kErrorNone;