Add tooling to help build metric DataRequirements

## The problem
As a metric developer, it may be hard to verify that the correct data requirements are being specified without running a metric.
We should have some tooling to make that easier.

Perhaps with this debugging we might need to target a "fake" data catalog which is more consistent with the real world. Perhaps a subset of the Google CMIP6 data catalogue with some tweaks would be sufficient. I don't think it includes parent branching times etc, but we can either fake those or extract that info.

## Definition of "done"
* [ ] CLI entry to debug the data requirements of a single metric
* [ ] Supports using a fake data catalog instead of the currently ingested datasets

## Additional context