Expose additional sidecar configuration on cluster level

• Which image of the operator are you using? registry.opensource.zalan.do/acid/postgres-operator:v1.8.0
• Where do you run it - cloud or metal? Kubernetes or OpenShift? [Azure AKS]
• Are you running Postgres Operator in production? [yes]
• Type of issue? [feature request]

We are using sidecar containers for log exporting/processing on a per-cluster basis and have noticed a disparity between the configuration options available on the operator level (exposing a full k8s container spec) and cluster one (exposing a custom sidecar type).

In our particular case, we would like to override the command and args for a sidecar container specified on cluster level, which is currently not exposed.

I'm happy to provide a PR as adding just these extra values to the custom configuration type should be relatively trivial, however wanted to check first whether that was the preferred approach or whether we should look into refactoring the cluster sidecars to support a full container spec as well.

From a previous PR introducing the global sidecar container spec I pick up a tendency to support custom types as they're less error-prone to configure for users (and also require less changes to add our desired additional settings), however I'm open to try supporting a full spec as well if that's the preferred direction.

Start by comparing the cluster sidecar type in pkg/apis/acid.zalan.do/v1/postgresql_type.go with the operator-level container configuration in pkg/apis/acid.zalan.do/v1/operator_configuration_type.go. Clarify whether cluster sidecars should gain command and args or support the full container spec, then verify the chosen API behavior with the relevant operator tests and Kubernetes cluster configuration paths.