Topology aware scale-from-zero running of NSEs in response to NSC demand: Add networkservice chain element that notifies if there no active connections

[denis-tingaikin]

Overview

We need to shut down all supplied nse that had scaled from zero. For this, we need to add a chain element that can notify about no connection during some period.

Implementation details

1. Add new networkservice https://github.com/networkservicemesh/api/blob/main/pkg/api/networkservice/networkservice.pb.go#L251-L254 Server element into networkservice/common pkg
2. The element should expect two parameters: timeout, notifier. Note: these parameters have default values. For timeout it is 10m. For notifier it can be exit 0.
3. Implement count logic for requests and closes.
4. If there no request and no active connections during some peridod or no request from start then call notifier.
5. Add unit tests to cover.

References

#892