add PVC and configmaps support to kubernetes backend

[insatomcat]

Following #5 and #6, new try to add PVC and configmaps features to kubernetes backend.

It could be used that way in shinyproxy :

      - id: app1
        display-name: app1
        description: kubeapp...
        container-image: gcr.io/$PROJECT/blabla:latest
        container-cmd: ["R", "-e","shiny::runApp('.', port = 3838, host = '0.0.0.0')"]
        container-volumes: ["claim_pvcname:/data", "configmap_nameofconfigmap:/app/config/"]

[LEDfan]

Hi @insatomcat

Thank you for your PR!
In order to not have to implement every Kubernetes feature in ContainerProxy, we decided to add a generic way of adding Kubernetes specific configuration to a ContainerProxy app. This feature is called kubernetes-pod-patches and allows to patch the specification of the Pod before it is created by ContainerProxy.
More information about this feature can be found here (you have to scroll down a bit).
This should cover your use-case. Feel free to open an issue if you experience any issue with this feature.
The change is included in the just ShinyProxy 2.4.0

Again thanks for your contribution.