Transport layer and general implementation guidelines

Historically, Inventory service has only supported gRPC protocol for talking to its plugins. RESTful support was added in May, 2019. All objects, fields and types between both options are named as identical as possible. There is a complete functional parity between both options, although we believe gRPC is a superior choice of the two as it provides better latency and requires less JSON parsing overhead. On the other hand, RESTful option has lower entry barrier as majority of engineers are already familiar with REST development paradigms.