cp: Add BGP withdrawal support

Add support to withdraw the announced BGP prefix if the service isn't healthy. A service is considered healthy if the number of healthy pooled realservers meets the withdraw threshold (a withdraw theshold of 0.0 disables this feature and setting it to 1.0 requires all servers to be healthy to announce the BGP prefix)

If two or more services share the same BGP prefix, the first one that considers the service unhealthy will withdraw the BGP prefix, effectively depooling all services sharing the same BGP prefix.

Bug: T389216