[FR][DAC] Consideration: expose a callback function within `kibana export-rules` to organize the output

**Note**: this work will target the [DAC-feature](https://github.com/elastic/detection-rules/tree/DAC-feature) branch

related to #3407

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**This is a consideration and not a commitment to work at this point**. We should review the feasibility and supportability. If it is doable and not deemed maintainable, we can move the example code to the DAC reference for users to manually implement.

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Many users organize the rules in a specific manner, but the `kibana export-rules` command downloads them flat. There have been requests to add a callback function to pass in code specifically to organize the save.

IMO, I don't know if this is worth the work. While it would be easy to implement, the onus should be on the implementer. An easy resolution to this problem would be to:
* store it to a temp directory as flat
* call your own function on the flat output to organize accordingly