health_checks: support multiple health checks on the same cluster

[ggreenway]

Fixes #3642

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🐱

Caused by: #44255 was opened by ggreenway.

see: more, trace.

[ggreenway]

@wbpcode this is still a draft, but since you did some work in this area in #16391 a long time ago, I wanted to get your input on the overall design. This isn't ready for a close code review yet.

This design tries to change as little as possible when only a single health check is configured, because I think that will always be the majority use case, and I don't want to introduce bugs for the existing code.

Can you look at my changes to the envoy interfaces for this, and how I used a proxy object HealthCheckerGroup, and tell me if you like that design or not?