Add parameter to add a custom initContainer or sidecar

[m-k8s]

We would like to have a parameter that allows us to add an initContainer.

Why? Because we have custom agent configurations stored in different configmaps in different namespaces and we want to create a configuration generator script for the datadog agent.

Describe what you expected:
A parameter (List of containers) in the Values ​​to add an initContainer or a sidecar.

An example can be found on Bitnami's helm charts: https://github.com/bitnami/charts/blob/45cada10883f6ac5ad4ecaadf3c0d6aa1d9912bb/bitnami/postgresql/values.yaml#L634

Steps to reproduce the issue:
Download the latest version of the Helm chart, you will see that it is not possible to add an initContainer or a sidecar.