ENH: more useful table export find func

Summary

1. Add support for passing callable to BaseTableExporter(find_func=callable)
2. Add BaseTableExporter.all_keys.

• Needed for https://git.geomar.de/dm/bis/indiscale/sample-management/-/merge_requests/8

Focus

The magic happens in BaseTableExporter._call_find_function.

Test Environment

Test with geomar setup

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• All automated tests pass
• Reference related issues
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• Up-to-date JSON schema (or not necessary)
• Appropriate user and developer documentation (or not necessary)
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CHANGELOG.md

@@ -8,6 +8,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added ###
 
+* Added support for passing callables as `find_func` to the `BaseTableExporter`.
+* Added member `BaseTableExporter.all_keys`
 * Parsing from YAML now allows to give an existing model to which the YAML data model shall be
   added.