Restriction of Data Stream Type

[rustmaker]

Hi Confluent Team,

I have a issue with 'data.stream.type' It was implemented with enum and It's also using for index template. I would like to use custom index template because I have logs and metrics index template in ES Cloud but I would like to change my index template like kafka and I'll define index lifecycle policy etc. Actually I forked and I reimplemented this field without Enum. I would like to ask about why did you implement it as enum and why is not generic?