[tonic-xds] Add the loadbalancer tower service#2607
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YutaoMa
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YutaoMa
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Add load balancer tower service that's extensive to support outlier detection and customized channel pickers.
Issue: #2444
Motivation
tonic-xds currently uses tower::balance, which is hard-coded to using P2C LB algorithm. For xDS, we need more flexibility and need full control over the underlying channels so that we can implement customized LB, outlier detection, etc.
Solution
Leverage the channel states (Idle, Connecting and Ready for now), build a new LoadBalance tower service whose poll_ready() handles channel state transitions and call() handles load balance picking algorithm.
Also, added a simple ChannelPicker trait to abstract channel picking algorithm. In the future, customized LB can implement this trait and tonic-xds will provide a way to register customized LB and make it available for LoadBalance layer.