Add sidecars and initcontainers

[samof76]

Fixes #187

Changes proposed on the PR:

• Adds sidecar
• Adds initcontainer

[ese]

Thanks @samof76, what is the use case you are trying to resolve with this feature? It could be something that we should abstract and manage directly from the operator.
Anyways I'm not against the feature but I would name the sidecars property as extraContainers

[samof76]

We require addition containers for

• manage failover to be propagated to the DNS
• manage periodic backup of the RDB files to blob storages
• Some other additional tasks

[samof76]

@ese sidecars -> extraContainers

[ese]

Thanks @samof76, could you resolve conflicts and generate CRDs with changed spec?