Critical Description
We are rolling out functional index with 1.0 as a new feature.
One can use sql to create the index. But we don't have a way to create one via spark-ds writes.
So, there is no real purpose of exposing "hoodie.metadata.index.functional.enable" to end user. in other words, what incase someone enables this via spark ds w/o setting any other config.
lets revisit similar config we have for secondary index and fix both together.