More resilience to initContainers overrides

[burmanm]

What is missing?

Currently, overriding the "server-config-init" caused the "server-base-config" init container to be absent as well, making users unaware that the upgrade from 4.0 -> 4.1 would cause issues in starting the container. This caused also the k8ssandra-operator to fail when using 4.1.x and medusa.

We should allow automated correction to always include both server-config-init and server-base-config, but also allow user to add extra initContainers before and after these configs. "server-config-init" should be appended as last one, "server-base-config" before it.

Why is this needed?

Users might not be aware that all the required initContainers must be defined if one is overridden.