Add the original URL to the auth request

[killfill]

It would be nice to send the original URL to the auth request.

This would be useful in cases where the /auth request is on a different domain as the service thats behind it.

Thanks!

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[k8s-reviewable]

This change is 

[killfill]

signed cla

[aledbf]

/lgtm

[aledbf]

@killfill Thanks!