Allow custom ID for a subplot

Hello Implot-Team!
Currently each plot of a subplot will have a [PushID with its index](https://github.com/epezent/implot/blob/7b9171569bfe846aa1049a70bc599ad44ef0e5c1/implot.cpp#L2380). There should be a way to set a custom value for this.
In my case I have options to show/hide/reorder subplot items and therefore the same plotted metric ends with a different ID path depending of the user choice. When the user changes the layout then the plot state is lost because its ID path has changed.