Allow tenant specification in jsonnet ScaledObjects

[zalegrala]

What this PR does

Without this change, the ScaledObject created by enabling the autoscaling in the jsonnet expects to be able to query without a tenant. In environments which require a tenant for query, this becomes a challenge.

Here we include a config to allow the specification of a tenant, which when non-empty, will add the appropriate header during query to allow the Keda ScaledObject to query the needed metrics to make autoscaling decisions.

Which issue(s) this PR fixes or relates to

Fixes #

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]. If changelog entry is not needed, please add the changelog-not-needed label to the PR.
• about-versioning.md updated with experimental features.

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Note

Enables tenant-scoped Prometheus queries for KEDA autoscaling.

• Adds _config.autoscaling_prometheus_tenant; when non-empty, sets customHeaders with X-Scope-OrgID on prometheus triggers in autoscaling.libsonnet
• Updates CHANGELOG.md to note the enhancement

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