Make PollingInterval configurable in HTTPScaledObject

[Spenhouet]

Provide the PollingInterval of the ScaledObject as parameter in the HTTPScaledObject.

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Fixes #1251

[wozniakjan]

lgtm, thank you!

ptal @zroubalik, I know you are not a big fan of duplicating fields between HTTPScaledObject and ScaledObject but imho this one also makes sense because the operator sets this to a hardcoded 15s and patches over any custom changes to the ScaledObject made by the user

[zroubalik]

Alternate proposal:

What if we just remove the hardcoded 15s and let it on ScaledObject?

This project is still beta so backwards compatibility is something we can ingore for certain cases and I think this one is a good one.

[JorTurFer]

The add-on is connected to KEDA as external-push scaler, so the polling interval isn't so relevant indeed.
When there is a change on the "active" state, the add-on pushes the change automatically, so the activation/deactivation is notified instantly. About using the polling interval to cache metrics, I don't think that HTTP Add-on metrics should be cached

I don't have in my mind all the add-on details atm, but I think that it's more or less how I have described.

Which is the goal of changing the pollingInterval value?