CSDS dumping broken by xDS fallback

[ejona86]

grpcdebug expects only a single ClientConfig from FetchClientStatus, and fails with "Received unexpected number of ClientConfig" when more than one is found.

gRPC implementations of CSDS were enhanced by gRFC A71 to support fallback, and do so by setting client_scope on the ClientConfig. That requires returning multiple ClientConfig messages, one for each client_scope.

As seen when investigating b/394850611 after a grpc-java upgrade.

CC @markdroth, @dfawley