Support reading TiDBs information from multiple cluster

[YangKeao]

Development Task

• Add design doc for multi-cluster support. #1096
• Add configs related to multi-cluster support, and listen to multiple ports. #1097
• Add cluster manager to manager cluster-scoped topology. #1098
• Move the metrics reader into cluster-scoped struct. #1099
• Add port-based routing rules and groups. #1100
• Route PD and backend traffic through cluster DNS. #1101