Allow the use of a secret located in a different namespace

[aledbf]

What this PR does / why we need it:

In the docs we mention that ns/secret is allowed but not in the code https://kubernetes.github.io/ingress-nginx/user-guide/nginx-configuration/annotations/#authentication

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged):

fixes #3493

[ElvinEfendi]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ElvinEfendi

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[kfox1111]

This should have a flag to enable this functionality? Normally I don't want users to be able to reach across namespaces for secrets.

[aledbf]

This should have a flag to enable this functionality? Normally I don't want users to be able to reach across namespaces for secrets.

This change only applies for the auth annotation and it's something already present. This PR just fixes a bug, not adding new features.

[aledbf]

/shrug

[zeph]

@aledbf I'm on ingress-controller 0.27.1 and your patch populated sns for me, awesome, but still this feature is not working... i'm getting a no object matching key "<ns>/<secret>" in local store

[aledbf]

@zeph please open a new issue. It would be great if you could include the steps to reproduce it.

[zeph]

I copy pasted from here... https://kubernetes.github.io/ingress-nginx/examples/auth/basic/ xD

ok, I will...

[aledbf]

@zeph just in case, I cannot reproduce what you describe following the example
https://gist.github.com/aledbf/4298bacb1487bf8084d4b2577dfb0a24

Edit: please make sure you are using the latest version