Support d2 symlink node in indis flow

[bohhyang]

Background

We need to support D2 symlink (d2 single master/primary feature, wiki) in INDIS flow.

The current ZK flow handles symlinks implicitly in SymlinkAwareZookeeper, which captures the data of a symlink node, immediately get the data of the actual master node, and send the fetched data to the symlink node callback. It can't be shared with INDIS flow.

For symlink clusters, the event bus subscribers, like "ClusterLoadBalancerSubscriber", "UriLoadBalancerSubscriber" subscribe to the symlinks ($FooClusterMaster), instead of the actual master node (FooCluster-prod-ltx1), we need to publish to event bus under the symlink names.
For other clusters, we need to publish under its original name. Note that these clusters could be either:

1. regular clusters requested normally.
2. clusters that were pointed by a symlink previously, but no longer the case after the symlink points to other clusters.

Changes

Subscribe to and handle d2 symlink node data in xDSClient and the xDS-to-d2 data adaptor.

Test Done

build and unit tests.
Verified with Toki qei deployment that the normal service, cluster, and uri nodes can be fetched successfully.
Will need to test fetching the symlink node with d2-proxy after release (due to SI-34767, the snapshot can't build with d2-proxy).