feat: ACL labels from environment variables (steveiliop56#422)

* feat: add LabelService to retrieve application labels from environment variables

* feat: allow usage of labels from docker and env variables simultaneously

Prioritize labels from environment variables over labels from docker
labels

* fix: handle error returned by label_serive.go/LoadLabels

see steveiliop56#422 (comment)

* refactor(label_service): use simple loop instead of slices.ContainsFunc to avoid experimental slices package
see steveiliop56#422 (review)

* refactor: merge acl logic into one service

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Co-authored-by: Stavros <steveiliop56@gmail.com>