feat(fluent-operator): add image reference by digest

[Boojapho]

Related to #356

Add ability to reference images by digest to fluent-operator Helm chart. Digest will override tag.

[benjaminhuo]

For PRs of the fluent operator chart, we recommend to submit to https://github.com/fluent/fluent-operator/tree/master/charts first, we'll sync each fluent operator release chart here in this repo