feat: Add onClusterManager to Clustering to allow easy default overrides (e.g. setting minClusterSize)

[darronschall]

When using the default cluster renderer, allow callers to easily configure the minClusterSize value.

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Fixes #388 🦕

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[Jasperav]

I actually think it's easier to add a lambda to the current method which can look like this:

modifyDefaultClusterRenderer: (DefaultClusterRenderer) -> Unit = { }

and than users can just do whatever they want with it, so you add 1 method to the existing method and call the lambda after creating the default cluster renderer.