Make config fields nullable

[Kahbazi]

Fixes #531

[Tratcher]

I had meant for this to cover the config model types consumed through the IProxyConfigProvider (Cluster, ClusterProxyHttpClientOptions, etc.), I hadn't considered the IOptions<T> types like ActiveHealthCheckMonitorOptions. Thanks for being thorough 😁.

Having reviewed those IOptions types now, it looks like most of them were already intended to provide fallback defaults for fields from the config model. These types are also used directly by implementations, where an alternate implementation would likely provide its own options type. I think it's fine for these to keep their non-nullable default values. Please revert changes for these options and their consumers:

• ActiveHealthCheckMonitorOptions
• ConsecutiveFailuresHealthPolicyOptions
• TransportFailureRateHealthPolicyOptions

[Tratcher]

FYI: Conflicts with #600. We'll see which merges first.

[Tratcher]

Thanks