Degraded health state

Currently a destination's health may be marked as Unknown, Unhealthy, or Healthy.

ASP.NET Core also supports a Degraded state:
https://github.com/dotnet/aspnetcore/blob/a31f34e8a1618210dc47675d667180765203cdfb/src/HealthChecks/Abstractions/src/HealthStatus.cs#L30

This might be useful when filtering and prioritizing destinations for traffic. E.g. a degraded destination may be sporadically returning errors, but that's still better than an unhealthy endpoint that's unreachable.

Start by reading the linked ASP.NET Core HealthStatus definition, then trace YARP's destination health representation and the code that filters or prioritizes destinations for traffic. Done means destinations can represent the Degraded state and the existing health-based selection behavior can distinguish it from Healthy and Unhealthy.

Written by the indexing model from the issue text.