why not support the exportTo annotation, to make services can be discover across namespaces

[andrewshan]

What is the feature you want to add?
support service discovery can across namespaces, ex, one can discover some common services not in same namespaces.

Why do you want to add this feature?
Many service rpc framework has the namespace isolated features, services in one namespace cannot directly invoke the services in other namespaces. But for some common services, such as storage, cache, gateway services, need to be invoked by services in different namespaces. So we need some mechanism to make user can do it fluently.

How to implement this feature?
services, namespaces can add the exportTo annotation, exportTo="*" make it can be discovered by all namespaces, exportTo="targetNamespace" make it can be discovered by the specific namespacce.

Additional context
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