F auto converter and transformer registry

Summary

Part of https://gitlab.indiscale.com/caosdb/customers/horizon-batcat/horizon-batcat-meta/-/issues/150. Makes the behavior of scanner.scan_structure_elements consistent to scanner.scan_directory by auto-generating converter and and transformer registry from the crawler definition.

Focus

Just the new optional behavior. Since it's still ok to give both registries explicitly to reproduce the former behavior, we shouldn't lose any backwards compatibility.

Test Environment

Unit test and manually run the elabFTW crawler from the elabFTW showcase.

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