feat(nats-jetstream): Add leaderOnly parameter

[jpig18]

Add leaderOnly parameter to query only stream leader

This adds a new optional 'leaderOnly' parameter to the NATS JetStream scaler that appends '&leader-only=true' to the monitoring URL when enabled. This is useful in clustered NATS deployments to ensure metrics are fetched only from the stream leader node.

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Checklist

• When introducing a new scaler, I agree with the scaling governance policy
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Fixes #7322

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[JorTurFer]

Hello
Currently KEDA only checks metrics from leader because only the leader has all the visibility of on-going messages and pending messages. I think that this is a nice improvement because current logic is more complex just to check which is the leader (I didn't know about that parameter tbh and it solves the discovery logic)

I mean, I see the benefit of this change because we can remove logic in place to achieve the same, but it's not something that we need to expose to users because it's the current behaviour. Do you know when the support for that parameter was added?