HealthCheckerConfig should validate it has only 1 set field

As discussed in https://reviews.apache.org/r/42816/ - the HealthCheckerConfig struct has 2 fields, but these 2 fields are treated like a union today behind the scenes. This union-style treatment is silent - we check for a shell 1st and use it and ignore http if also populated. This could lead to user confusion. It would be better to fail fast if both types of health check are configured.