Support displaying multiple partitions on one instance in cluster info

[mornyx]

Feature Request

As a continuation of work on https://github.com/pingcap/tidb-dashboard/issues/1625. We should support showing multiple partitions on an instance. The current data model only supports one single partition (both BE & FE):

type InstanceInfo struct {
    Type           string `json:"type"`
    PartitionPathL string `json:"partition_path_lower"`
}

type Info struct {
    Host        string           `json:"host"`
    // ...
    // Instances in the current host. The key is instance address
    Instances map[string]*InstanceInfo `json:"instances"`
}

[mornyx]

/assign