Healthcheck handler should support returning any InstanceInfo Status

[NiteshKant]

Current HealthCheckCallback only supports a binary status which translates to UP and DOWN status for InstanceInfo.
In order to have healthcheck status not used in STARTING state, eureka client does not invoke the healthcheck callback when the status is STARTING.
This forces applications to spread the instance status update in two places:

• HealthCheckCallback giving UP/DOWN status.
• Application code transitioning status from STARTING to UP

This change will provide a new interface (deprecating the old HealthCheckCallback interface), that will provide the InstanceInfo.InstanceStatus as its return value. This will enable applications to have the instance status update logic in one place i.e. the HealthCheckHandler