Add the ability to put items in an expander by annotating Pydantic model fields

[madkopp]

This pull request adds the ability to put items in an expander by annotating fields of a Pydantic model with an Expander annotation. An example of this is also added to the examples directory.

Addresses #24 but may not fully close it.

---

Contribution License Agreement

By submitting this pull request you agree that all contributions to this project are made under the MIT license.

[madkopp]

Happy to add any documentation or unit tests if you have a suggested format for that.

[madkopp]

@lukasmasuch