Let KEDA functions scale to zero

[Tushar-Khandelwal-2004]

Title: Let KEDA functions scale to zero by default

Why

The KEDA deployer currently defaults minReplicaCount to 1 when generating the HTTPScaledObject. That keeps at least one function pod running even when there is no traffic, so functions cannot scale to zero by default.

KEDA's HTTP add-on supports waking the function back up through the interceptor, and users who want to avoid cold-start latency can still set scale.min: 1 explicitly.

Fixes #3924.

What changed

• Default the KEDA deployer's generated replicas.min value to 0.
• Keep honoring an explicit deploy.options.scale.min value.
• Add regression coverage for both the default and configured minimum replica behavior.

Testing

• go test -count=1 ./pkg/keda

[linux-foundation-easycla]

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Welcome @Tushar-Khandelwal-2004! It looks like this is your first PR to knative/func 🎉

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