[FEAT] Support multiple stacked overlays for "variant comparison"

[assynour]

Is your feature request related to a problem? Please describe.

In process mining, comparing multiple variants is a very common use case.

A variant refers to all the executions of the business process that share a specific feature (e.g. all executions of accepted loans, executions of the summer period, etc.).

As an analyst, I would like to be able to visualize and compare multiple variants. For example:

• Compare the average performance of summer vs winter sales
• Compare the average performance of standard shipping vs express shipping vs same-day delivery
• ...

As of v0.34.1 of bpmn-visualization, it is possible to add only one overlay per element position to visualize statistics.

Describe the solution you'd like

I want to be able to stack multiple overlays on the same position. Each overlay has its own style and label for displaying the statistics computed for one variant, similarly to the visualization proposed in #2711.

Additional context

This visualization is similar to the one described in #2711 but with a more practical use case.