clientv3: move health balancer to "balancer"

[gyuho]

Preparing balancer rewrite with new gRPC. #9106

[codecov-io]

Codecov Report

Merging #9462 into master will increase coverage by 0.04%.
The diff coverage is 87.05%.

@@            Coverage Diff             @@
##           master    #9462      +/-   ##
==========================================
+ Coverage   72.51%   72.56%   +0.04%     
==========================================
  Files         364      364              
  Lines       30874    30890      +16     
==========================================
+ Hits        22389    22416      +27     
+ Misses       6855     6854       -1     
+ Partials     1630     1620      -10

Impacted Files	Coverage Δ
clientv3/retry.go	94.86% <100%> (ø)	⬆️
clientv3/balancer/grpc1.7-health.go	86.33% <85.5%> (ø)
clientv3/client.go	84.74% <91.66%> (+0.97%)	⬆️
pkg/adt/interval_tree.go	81.68% <0%> (-6.91%)	⬇️
auth/simple_token.go	87.03% <0%> (-6.49%)	⬇️
clientv3/namespace/watch.go	72.72% <0%> (-6.07%)	⬇️
clientv3/txn.go	95.55% <0%> (-4.45%)	⬇️
etcdserver/cluster_util.go	76.64% <0%> (-1.46%)	⬇️
etcdserver/api/v2http/client.go	90.33% <0%> (-0.45%)	⬇️
proxy/grpcproxy/watcher.go	89.79% <0%> (ø)	⬆️
... and 16 more

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