[services] Tunnelbroker - Add messages request and confirmation method to grpc proto file

Summary:
This diff introduces the new gRPC method for the client checkpointing request to the server according to the ENG-1158 ashoat comment.
The purpose of this method is: the client sends the request to the tunnelbroker server to confirm successful checkpointing (the client successfully received and persisted the updates from the Tunnelbroker)

Related task: ENG-1306

Test Plan: gRPC protoc successfully generates the source code.

Reviewers: karol-bisztyga, palys-swm, varun, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phab.comm.dev/D4217