KeyConditionExpr

This sum type models:

1. partition key equality expressions
2. composite primary key expressions where sort key expression is equality
3. extended composite primary key expressions where sort key is not equality eg >, <, >=, <=, between, begins_with

Note 1), 2) and 3) are all valid key condition expressions used in Query DynamoDB queries BUT only 1) and 2) are valid primary key expressions that can be used in GetItem, UpdateItem and DeleteItem DynamoDB queries