Re-add `whenCreated` to MapContainer or allow class based React projects access to `Map` without hooks

[mikebrucker]

I am upgrading a company React project and we use class based components. Version 4 of react-leaflet does not allow access to Map without using the useMap hook. We need to be able to keep our code consistent and not be forced to use hooks.

Bug report in v4

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Please make sure to check the following boxes before submitting an issue.\ Issues opened without using this template will be closed unless they have a good reason not to follow this template.

• [x] All peer dependencies are installed: React, ReactDOM and Leaflet.
• [x] Using the latest version of React and ReactDOM v18.
• [x] Using the supported version of Leaflet (v1.8.0 minimum) and its corresponding CSS file is loaded.
• [x] Using the latest v4 version of React-Leaflet.
• [x] The issue has not already been reported.
• [x] Make sure you have followed the quick start guide for Leaflet.
• [x] Make sure you have fully read the documentation and that you understand the limitations.

Expected behavior

Access to Map is doable in class based components without hooks.

Actual behavior

In version 4 we are forced to use hooks.