Optionally enable init/sidecar containers

[sdudoladov]

The operator currently support running sidecar and init containers side-by-side with Postgres containers. For maintenance reasons , we would like to be able to enable/disable these features per k8s cluster. The task is to:

1. Add relevant toggles to the operator configuration (CRD and configmap) similar to existing boolean options such as enable_shm_volume.
2. Log a warning if a user attempts to use init/sidecars containers when this feature is disabled.

[redbaron]

IMHO it should be a hard error and no Postgres resources to be created

[sdudoladov]

we also may need to restrict the ability to re-write tolerations/timeouts in a similar fashion

IMHO it should be a hard error and no Postgres resources to be created

Good point 👍 Sadly in the present operator codebase there is no single place where we can validate all the (un)desired properties of the PG manifest, so to make it a hard error we can, for instance, put a separate check in the beginning of Sync and Create methods.