feat(helm): add podLabels attribute

[jperville]

This PR adds an optional podLabels attribute to the helm chart values.yml
The default value is an empty object to be compatible with the existing behavior.

Tests:

# if `yq` is not present, install from https://github.com/mikefarah/yq

# with the default/empty podLabels
helm template my-mailcrab . \
  | yq 'select(.kind == "Deployment").spec.template.metadata' 

# with a custom podLabels
helm template my-mailcrab . --set-json podLabels='{"my":"label"}' \
  | yq 'select(.kind == "Deployment").spec.template.metadata'

[jperville]

@marlonbaeten would it be possible to review this simple PR for mailcrab helm chart? Thanks a lot.