Support feature value of type map with scalar, non-string keys

**Is your feature request related to a problem? Please describe.**
Many features are of map value that have features that have non-string keys like timestamps or uuids. Think about map of purchases by transaction_id.

**Describe the solution you'd like**
I would like a new type that is a map with non-string, scalar key types.

**Describe alternatives you've considered**
We can convert non-string keys to strings but then there is no way to convert them back.