IN: The schema to query.
IN: Should range from 0 up to the count-1 returned in hc_schema_get_count.
OUT: Updated to point to a string that is an attribute name of one attribute in the schema.
OUT: Updated to be the type associated with that name in the schema. If the server schema references a type that the client library does not support, then the type is returned as HC_UNKNOWN_TYPE.