Feature Request: Support configurable panic threshold

[rexnp]

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Tell us about your request
Envoy today uses a default 50% panic threshold in each of its clusters. Panic threshold allows Envoy to fall back on routing traffic to unhealthy/degraded hosts if the overall percentage of healthy hosts within the cluster falls below this value.

Which integration(s) is this request for?
all

Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?
Effectively, panic threshold allows Envoy to override certain mechanisms like active health checks and outlier detection if half of the hosts within the cluster are unhealthy. Allowing panic threshold to be configurable means users have the option to further control whether and when a client should continue routing requests to potentially unhealthy endpoints or to stop sending traffic altogether.