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Geometry Nodes: support randomly picking instances from collection

This uses the "id" attribute to randomly pick instances from a collection
for each point.

There is one issue. When the collection is updated (e.g. when an object is
added to it), the nodes modifier is not automatically updated. It seems
like we don't have the infrastructure to support this dependency yet.
The same issue exists in the Boolean modifier and with collision collections.
This should be solved separately soonish.

When "Whole Collection" is disabled, one direct child of the input collection
is instanced at each point. A direct child can be an object or a collection.

Currently, all objects are picked approximately equally often. In the future,
we will provide more control over which point gets which instance.

Differential Revision: https://developer.blender.org/D9884

Ref T82372.