feature: new API and Proxy methods

[rustatian]

Reason for This PR

ref: https://github.com/roadrunner-php/centrifugo/issues/6

Description of Changes

Proxy:

service CentrifugoProxy {
        rpc SubRefresh(SubRefreshRequest) returns (SubRefreshResponse);
}

For the SubRefresh request RoadRunner will send a subrefresh request type in the metadata.

API:

service CentrifugoApi {
  	rpc Batch(BatchRequest) returns (BatchResponse) {}
  	rpc DeviceRegister(DeviceRegisterRequest) returns (DeviceRegisterResponse) {}
  	rpc DeviceUpdate(DeviceUpdateRequest) returns (DeviceUpdateResponse) {}
  	rpc DeviceRemove(DeviceRemoveRequest) returns (DeviceRemoveResponse) {}
  	rpc DeviceList(DeviceListRequest) returns (DeviceListResponse) {}
  	rpc PushUserChannelList(PushUserChannelListRequest) returns (PushUserChannelListResponse) {}
  	rpc PushUserChannelUpdate(PushUserChannelUpdateRequest) returns (PushUserChannelUpdateResponse) {}
  	rpc SendPushNotification(SendPushNotificationRequest) returns (SendPushNotificationResponse) {}
}

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.