toExistInDynamoTable assertion only works with strict PK/SK naming conventions

[JoshThorne]

If the table you are testing on does not have a partition key of 'PK' (i.e. instead of 'pk') or does not have a sort key of 'SK' an error is thrown/test is failed regardless of table state. Instead the assertion should allow the dev to input a PK/SK of their choosing.