[ENH] - Improve control over "keep alive" option

[kcpevey]

Feature description

In practice, the "keep alive" button is dangerous. It gives users the ability to incur ongoing, continuous costs on could deployments. It would be nice to be able to control who is allowed to keep apps alive.

The easiest (blanket) option here would be to disable keep alive for everyone on the platform. This can be done by exposing the keep_alive option in jhub-apps config. Follow on work would also be needed to configure this via the jhub deployment running jhub-apps.

A more fine-grained option would be to have jhub-apps fetch optional special permissions from the jhub deployment. This would require follow on work in the jhub deployments to set up these new permissions.

Value and/or benefit

More admin control over who can run continuous running apps and therefore more control over costs incurred by deployed apps.

Anything else?

No response