[K8SPSMDB-294] Add support for custom sidecar containers

[spron-in]

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Tell us about the feature Add support for MongoDB Operator to run custom sidecar containers and specify them in cr.yaml during creation.

Which product(s) is this request for? MongoDB, Operators

Tell us about the problem

• Problem 1: run custom monitoring exporter as a sidecar container
• Problem 2: customizable service mesh with sidecar containers

Describe alternatives you've considered I cannot think of any sustainable workaround here - only manual hack of k8s objects.

Additional context None

[spron-in]

The feature was released as part of 1.7.0 version of the Operator. See release notes here.