config_addon.py and config.py should merge, not override, custom_types keys.

[shilohbeeler-NI]

Description of issue

As encountered in #1485, if there exist custom types in a module's config.py metadata and in the config_addon.py metadata, the config_addon.py will override config.py and only use custom types from config_addon.py in the code generation. These two should be merged instead to allow custom types which rely on C Structs and completely new, python-only custom types to both exist in the generated code.