Co-authored-by: Michael Marshall <mmarshall@apache.org>
### Motivation
There was a suggestion [in a dev mailing list discussion](https://lists.apache.org/thread/bgkvcyt1qq6h67p2k8xwp89xlncbqn3d) that the Helm chart's appVersion should be used as the default image tag.
### Additional context
There are some limitations in Helm. It is not possible to set "appVersion" from the command line. There's in an open feature request https://github.com/helm/helm/issues/8194 to add such a feature to Helm.
### Modifications
- change default values.yaml and set the tags for the images that use the Pulsar image to an empty value
- add "defaultPulsarImageTag" to values.yaml
- add a helper template "pulsar.imageFullName" that contains the logic to fall back to .Values.defaultPulsarImageTag and if it's not set, falling back to .Chart.AppVersion
- use the helper template in all other templates that require the logic
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