ad-hoc client generation for CRDs

[aespinosa]

Only works and tested in java as it's what we are using at the moment.

Introduces the following variables to a setting file:

• KUBERNETES_CRD_MODE - must be set to true to activate the maven profile.
• OPENAPI_MODEL_LENGTH - customizes how much prefix of the generated model to strip. Similar to what is done in https://github.com/kubernetes-client/java/blob/master/docs/generate-model-from-third-party-resources.md#generate-java-model-from-the-downloaded-openapi-spec through the -l<n> flag.
• KUBERNETES_CRD_GROUP_PREFIX - the prefix of the apiGroup to filter for the CRDs

And similar to https://github.com/kubernetes-client/java/blob/master/docs/generate-model-from-third-party-resources.md, OPENAPI_SKIP_FETCH_SPEC is needed.

Added codepaths in preprocess_spec.py to address kubernetes-client/java#921 by replacing the whole ListMeta tree with $ref: #/definitions/v1.ListMeta

Addresses #121

cc @brendanburns @yue9944882

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[aespinosa]

/assign @brendanburns

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[aespinosa]

/assign @mbohlool

[brendandburns]

/lgtm
/approve

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[yue9944882]

this is awesome! let me adjust the guide doc on the java repository.