Add config options to manage variables expansion

[irizzant]

What this PR does:
This PR adds support for specifying config options to manage variables expansion in a single point.

For example, to enable S3 storage backend simply add the following to the _config object:

...
local envVar = k.core.v1.envVar,
...
variables_expansion: true,
variables_expansion_env_mixin: [
          envVar.withName('S3_ACCESS_KEY')
          + envVar.valueFrom.secretKeyRef.withKey('S3_ACCESS_KEY')
          + envVar.valueFrom.secretKeyRef.withName('minio-secret'),
          envVar.withName('S3_SECRET_KEY')
          + envVar.valueFrom.secretKeyRef.withKey('S3_SECRET_KEY')
          + envVar.valueFrom.secretKeyRef.withName('minio-secret'),
        ],

where S3_ACCESS_KEY and S3_SECRET_KEY are taken for a secret named minio-secret.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[joe-elliott]

nice addition. just asking for some minor cleanup to the changelog.

[joe-elliott]

@irizzant CI was broken in main. If you could rebase/merge main CI should pass and we can get this merged.

[irizzant]

@joe-elliott that makes sense...I updated the branch as you suggested it should be ok then

[joe-elliott]

Nice. Thanks for the contribution!