Add better secret support

[valkum]

Is this a request for help?: No

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Is this a BUG REPORT or FEATURE REQUEST? (choose one): Feature Request

Add support for different secrets like S3 credentials for non AWS platforms and document ways to retrieve them in pipelines (normal steps and docker plugin steps).

If I understand the current code correctly, only a handful of custom settings are currently stored as k8s secrets.
Other secrets can only be added as ENV vars.
Ideally you could add all kind of different vars as secrets to k8s.

Additionally, it would be helpful for beginners to document, how to utilize the k8s secret store in pipelines.