Support thresholds in widget_billboard for newrelic_one_dashboard

[mkatana-silkycoders]

Feature Description

Please add support for threshold blocks in widget_billboard for the newrelic_one_dashboard resource (Terraform provider). Thresholds are available on other widget types (e.g., widget_table, widget_line), but not on billboards. This would let us keep the billboard visualization while showing simple status/SLI traffic lights (green/red) directly on the widget.

Describe Alternatives

• Use widget_table or widget_line only to get thresholds, even when a billboard view is desired.
• Move visual thresholds into alert policies/conditions, losing the per-widget cue.
  Both options are suboptimal: they force a different visualization or split logic between dashboard and alerting.

Additional Context

Attempting threshold inside widget_billboard yields Unsupported block type "threshold" from the provider. The current docs do not list thresholds for billboards. Adding this would simplify SRE dashboards needing quick “traffic-light” indicators for key SLIs without changing widget type.