This is the OSS portion of enterprise PR 2460.
Introduces a server-local implementation of the proxycfg.ResolvedServiceConfig
interface that sources data from a blocking query against the server's state
store.
It moves the service config resolution logic into the agent/configentry package
so that it can be used in both the RPC handler and data source.
I've also done a little re-arranging and adding comments to call out data
sources for which there is to be no server-local equivalent.
