Allow Custom Init Containers

[samzph]

The Problem
Currently, the automatic creation of topics must be done through a secondary pipeline or something that waits for the system to be ready. It also does not cover for if the data is lost due to command line deletion. For example, if I delete pulsar resources I will lose my tenants/namespaces/etc.

My Proposed Solution
If I had an initcontainer, like on pulsar-toolset or the brokers, I could run the bin/pulsar-admin tenants create and create my namespaces under the tenant. I'm not sure which container(s) store the actual Pulsar tenants data, so whichever that one is, it would be nice if on initial startup I could configure it to create resources.

Describe alternatives you've considered
Kubernetes Jobs would work, but if the install crashes or loses the data, there would have to be a flag or something to run the job. This may be possible but requires additional research on my end.

Additional context
A nice implementation of this can be seen in the Camunda Helm Chart
https://github.com/camunda-community-hub/camunda-helm/blob/3228ff19e168a9b1100852d480df2d8682be432d/charts/camunda-bpm-platform/templates/deployment.yaml#L36-L39