add ability to extend configuration.py

[rsmithcooper]

This gives the ability to extend the configuration.py file populated by the configMap.

Use case: adding an Okta OIDC integration using the social-auth library.

current workaround: editing the configmap manually after helm install and then redeploying the deployment

[bootc]

I don't have an issue with adding a mechanism for adding Python config code, but you shouldn't need this to add static configuration values—you can do that with the extraConfig setting. Do you need to be able to run actual Python code rather than just setting static values? If so you'd probably be better off using extraVolumeMounts and extraVolumes to mount your Python script into /etc/netbox/config/...