helm: Add configmap ref support for mem resolver

[wallyqs]

Adds support for referencing a configmap to be used as a memory resolver.

Usage (requires installed nsc):

curl -sSL https://nats-io.github.io/k8s/setup/nsc-setup.sh | sh
nsc generate config --mem-resolver --sys-account SYS > resolver.conf
kubectl create configmap nats-accounts --from-file resolver.conf

The reference the configmap as follows:

# deploy.yaml
auth:
  enabled: true
  resolver:
    type: memory
    configMap:
      name: nats-accounts
      key: resolver.conf

helm install nats-with-auth nats/nats  -f deploy.yaml

Signed-off-by: Waldemar Quevedo [email protected]

[ColinSullivan1]

LGTM.