feat: support standalone server access

[andrewshan]

What is the feature you want to add?
sdk can use the inputed server address to do all operations, no need to relay on the polaris.discover and polaris.healthcheck

Why do you want to add this feature?
when user use sdk with the standalone server, it won't register the polaris.discover and polaris.healthcheck

How to implement this feature?
in sdk side, when user not input the service address, it won't connect to the service

Additional context
Add any other context or screenshots about the feature request here.