Extract between non-main sub-apps

[therealbnut]

Objective

This is work in progress, just intended to show a general approach. It's a small extension on the existing SubApp::{extract, set_extract, take_extract} methods to have labelled versions of them. This allows for extraction between SubApps that aren't main.

One issue with this currently is it does not allow a deterministic order between the apps, which may mean you can't construct a DAG of extractions between subapps. This could be solved by storing the labels vector from SubApps::update, but that needs further thought.

This is mainly to provide to context when asking whether the general approach of adding more extract methods to SubApps would be accepted, or if it would still be blocked by the greater multi-world work.

Related issues:

• First Class Multi-World Bevy #17937
• Multi-world support #7893
• Multiple worlds for isolated simulations #2237

Solution

• Describe the solution used to achieve the objective above.

Testing

• Did you test these changes? If so, how?
• Are there any parts that need more testing?
• How can other people (reviewers) test your changes? Is there anything specific they need to know?
• If relevant, what platforms did you test these changes on, and are there any important ones you can't test?

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