Watch the resource group changes

[nolouch]

Feature Request

ref #5794
Provide a way let the client can watch the configuration changes, then TiKV/Storage can updates the priority settings.

Option

I prefer provide a common mechanism, that's client can watch different key or keyprfix in etcd, when it changes, the client can watch it. A basic implement in befor: #4308

Note:

• the config truth is the configuration stored in configuration server (Now is pd)
• the client(TiKV/TiFlash Nodes) watch the configuration and updates the changes in memory, affect the local config. (I suggest not rely on config file, the client only use it in memory)

[HuSharp]

/assign