Add jupyterhub-idle-culler module to the JH image

[4n4nd]

Is your feature request related to a problem? Please describe.
We have a public instance of Jupyterhub and sometimes users forget about their running servers which blocks our hardware resources even when they are not being utilized.
Related https://github.com/operate-first/SRE/issues/229

Describe the solution you'd like
We would like the jupyterhub-idle-culler module be included in the Jupyterhub image, so that the pod culling service can be set up from the jupyterhub-config.py configuration.

Describe alternatives you've considered
Current Alternative is to deploy the culling service as a separate deployment.

[HumairAK]

Example of how this would be used as a JH managed service: https://jupyterhub.readthedocs.io/en/stable/reference/services.html#launching-a-hub-managed-service