Callback collection through entry points

[awaelchli]

What does this PR do?

Fixes #9741

Adds support for Callback collection through Entry Points. See the added docs for a summary of what this feature does.

TODO:

• tests
• discuss api, verify terminology

Does your PR introduce any breaking changes? If yes, please list them.

No.

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Did you have fun?

I made sure I had fun coding 🙃

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