Add layer_time field to LayerOptimized message

Introduces a new float field 'layer_time' to the LayerOptimized message in Cura.proto to represent the time required to print each layer. Updates the field numbering to accommodate the new field.

Author: HellAholic

plugins/CuraEngineBackend/Cura.proto

@@ -100,8 +100,9 @@ message LayerOptimized {
     int32 id = 1;
     float height = 2; // Z position
     float thickness = 3; // height of a single layer
+    float layer_time = 4; // Time to print this layer in seconds
 
-    repeated PathSegment path_segment = 4; // layer data
+    repeated PathSegment path_segment = 5; // layer data
 }