qhub-config.yaml as a secret

[danlester]

Closes #902

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Changes:

Adds qhub-config-yaml secret containing a copy of the YAML file. Also mounts it on extension pods (optionally)

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• Bug fix (non-breaking change which fixes an issue)
• [x ] New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features to not work as expected)
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Testing

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