Is your feature request related to a problem? Please describe.
When deploying Loki in Simple Scalable Mode using the Helm chart, you can attach a sidecar container to the Write and Read components, however, you cannot do so with the Backend component.

Describe the solution you'd like
The Loki Helm chart should support adding a sidecar container to each of the Simple Scalable Mode components. In the past, there were PRs to add this capability to the Write component and Read component. I'd like to complete this by creating a very similar PR for the Backend component.

Describe alternatives you've considered
We could patch the StatefulSet directly, but we would like to be able to make use of the Helm chart and future improvements to the Helm chart.

Additional context
Previous MRs:

• Add extraContainer to Read pods
• Add extraContainer to Write pods