[ENHANCEMENT] Provide username for external services via Kubernetes Secret

[j-lindner]

Describe the use case:
When I want to configure connection to an external elasticsearch, I have to specify the user in the Helm Chart via param global.elasticsearch.auth.username

Describe the enhancement/feature:
I want to be able to specify the username for Elasticsearch (in my use case, but probably for all external services) also via a Kubernetes Secret, e.g. via a pattern like

existingSecret: ""
existingSecretUserKey: ""
existingSecretPasswordKey: ""

[hisImminence]

Hey @j-lindner, thanks for the enhancement request!

Can you share some more motivation/ context to understand this feature request better? Like in what scenarios is global.elasticsearch.auth.username (plain value) insufficient?