Implement UPPER_SNAKE_CASE enum converter for control plane API

[firenero]

This change allows the use of PascalCase for enum values instead of UPPER_SNAKE_CASE used in DynamoDB API.

This is a breaking change for DescribeTable call. But since it's the only control plane endpoint supported yet and most likely not widely used, it's a good moment to introduce a breaking change to have a better API moving forward.