Routing: Export HealthCheckSettings

[meanwhile131]

Currently, trying to build a BurstObservatory config from conf.BurstObservatoryConfig:

config.BurstObservatory = &conf.BurstObservatoryConfig{
	SubjectSelector: []string{"out"},
	HealthCheck: &conf.healthCheckSettings{},
}

Results in a compiler error:

1. name healthCheckSettings not exported by package conf [UnexportedName]

conf.BurstObservatoryConfig is public, but uses conf.healthCheckSettings (a private struct), which prevents creating a conf.Config with a burst observatory from an outside package.
Making healthCheckSettings public should allow building of a config with a burst observatory.