xds: change cdsbalancer to use update from dependency manager (#8907) This PR is part of A74 changes. This PR changes following: 1. Changes the CDS balancer to use the CDS and EDS/DNS update from XDSConfig received by xds_resolver instead of starting its own CDS watchers. 2. Remove cluster resolver completely 3. CDS balancer creates the priority config and creates priority balancer as its child. 4. Remove serializer from CDS balancer since it was there to serializer watcher updates and ClientConn updates, now that there are no watchers and clientConn updates are already serialized , serializer is no longer needed. 5. Update tests in cdsbalancer package to check the priority config instead of cluster_resolver config. 6. xds_resolver tracks reference counts for weighted cluster and cluster specifier plugin seperately. xds_resolver subscribes to the weighted cluster in dependency manager when referenced for first time and unsubscribes when references in xds_resolver go to zero. 7. Incase of LDS/RDS resource error , we now directly send empty service config , instead of sending a complete service config with clusters that RPCs are still referenced to. 8. Minor test fixes. RELEASE NOTES: * xds: * Ambient errors for cluster resources are now logged exclusively in the dependency manager and are no longer propagated to Load Balancing (LB) policies. * When re-resolution is requested, all clusters of type LOGICAL_DNS will be re-resolved simultaneously, rather than only a single cluster. * Upon receipt of a listener or route resource error, all in-flight RPCs will now fail immediately. * Any error encountered during the creation or update of a priority configuration will now transition the channel to a TRANSIENT_FAILURE state.
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