pre processor / post processor support

[teyckmans]

The DSL resource generation should support pre-processors on the DSL types and post processors on the API types.

The pre-processors can be used to alter the DSL type before generating the API type instance. This can be used for e.g.:

• Automatically setting the namespace/name/... on resources (this may lead to the possibility to completely omitting the metadata block).
• Configuring resource requests/limits based on labels.

The post-processors can be used to check for resource completeness and the generated API type instance, e.g.:

• require health/liveness probes
• require at least cpu/memory limits