[nats helm] add chart-specific value support for imagePullSecrets

[walsm232]

This commit takes care of allowing a user to explicitly set imagePullSecrets at the NATS (StatefulSet) and NATS Box (Deployment) level instead of needing to do it via a global value.

Supporting this like it is currently via only a global value could be problematic for a couple of reasons when this NATS chart is a dependency of another chart:

• when you define imagePullSecrets at the global level, it affects all dependencies using that Helm value. This may not be desirable if you have multiple dependencies, each needing different image pull secrets.
• users might need different image pull secrets for different dependencies or even within different components of the same dependency. By only having a global override, this fine-grained control is limited.
• managing imagePullSecrets for different dependencies solely through a global Helm value could lead to complexity and potential confusion.

Overview of scenarios with this potential change:

• global value is set and chart-specific values not set: global value will be used.
• chart-specific values are set and global value is not set: chart-specific values will be used.
• both the global value and chart-specific values are set: chart-specific values will be used.
• both the global value and chart-specific values are not set: imagePullSecrets does not get defined

[caleblloyd]

First off, thank you for contributing. I see your point, but we try to add as little additional schema as possible to this chart.

When a property only affects a single resource, we prefer to merge/patch it into place using the same schema as it's k8s resource. This should be achievable today using:

# merge a pull secret for nats pods, this will override the global setting
podTemplate:
  merge:
    spec:
      imagePullSecrets:
      - name: regcred

# merge a pull secret for natsBox pods, this will override the global setting
natsBox:
  podTemplate:
    merge:
      spec:
        imagePullSecrets:
        - name: regcred