Extend index-cardinality-detection to also cover hypothetical planned indexes

[vin-yu]

Extend index-cardinality-detection to determine if an index is high cardinality based on an index input.

• Developers and admins planning to build indexes can use index-cardinality-detection tool to determine if the index is high cardinality.
• This could also be used in CI/CD check point before an index change goes through into prod

[rawalpuneet]

Good idea Vin. shouldn't be a big change. I plan to call hypothetical index, planned indexes. will update the docs for that too.

[rawalpuneet]

@vin-yu / @tmcallaghan , can you review and see if the changes make sense to you in PR?