Consider moving SyncRegions to another gRPC service

[overvenus]

Currently, SyncRegions is part of PD service, but it is for internal use only. To avoid pollute client APIs, please consider moving it to another gRPC service.

Cc #1237

[disksing]

@overvenus I have a different opinion on this issue. Firstly it’s hard to say that it pollutes the client -- if you don't need it, you just don't call the method. The generated client won't force you to write any extra code to make it work. Secondly, this interface does not necessarily have to be used only internally by the PD. For example, third-party components can also use this method to synchronize all region metas from PD, which could be used for tasks like external scheduling or data analysis in the future.