Allow to explicitly disable resource requests or limits of Sophora Server

[hehu80]

If you want to run the Sophora Server in a Cluster without setting resource requests or limits, you have to "delete" the default values by setting for example .Values.resources.limits to null in the values.yaml.
Unfortunately, deleting of default-values using null is still not possible while using Helm with sub-charts (see helm/helm#12879).
Adding an enabled flag to disable resources settings would be great.

For example:

resources:
  limits:
    enabled: true
    memory: "20Gi"
    cpu: "1.0"
  ...

to configure the resource limits

resources:
  limits:
    enabled: false
  ...

to run the Sophora Server without resource limits.

[hehu80]

#130