Support Downward api as template variable

[josedev-union]

It is sometimes useful for acorn containers to have information about themselves.
Also, acorn app to have its information like appInstance name, namespace etc.

[djcarpe]

Description

There may be cases where we'd like containers to have access to pod information through environment variables.

Some example pod information that can be pulled through the downward api is the fieldRef and resourceFieldRef.

A short list of available fields

field	description
metadata.name	the pod's name
metadata.namespace	the pod's namespace
metadata.uid	the pod's unique ID
metadata.annotations['<KEY>']	the value of the pod's annotation named (for example, metadata.annotations['myannotation'])
metadata.labels['<KEY>']	the text value of the pod's label named (for example, metadata.labels['mylabel'])
spec.serviceAccountName	the name of the pod's service account
spec.nodeName	the name of the node where the Pod is executing
status.hostIP	the primary IP address of the node to which the Pod is assigned
status.podIP	the pod's primary IP address (usually, its IPv4 address)

Resources

Kubernetes Downward API: https://kubernetes.io/docs/concepts/workloads/pods/downward-api/

Example Kubernetes downward API usage: https://kubernetes.io/docs/tasks/inject-data-application/environment-variable-expose-pod-information/

Previous discussion in Acorn Slack: https://acorn-users.slack.com/archives/C03R9ME0SKC/p1663138625938179