static String collectionType(String baseType) {
        return baseType + "Collection";
    }
Get the concrete Java type of the field-- for example, ArrayList.

@param headerGenerator   The header generator in case we need to add imports.
@param structRegistry    The struct registry in case we need to look up structs.

@return                  The abstract java type name.
generator/src/main/java/org/apache/kafka/message/FieldSpec.java