Resolve "Refactor how PNDs are implemented in the code"

.gitignore

api/go/gosdn/networkelement/networkelement.pb.go

api/go/gosdn/networkelement/networkelement.pb.gw.go

api/go/gosdn/networkelement/networkelement_grpc.pb.go

api/go/gosdn/plugin/plugin.pb.go

api/go/gosdn/pnd/pnd.pb.go

api/go/gosdn/pnd/pnd.pb.gw.go

api/go/gosdn/pnd/pnd_grpc.pb.go

api/openapiv2/gosdn_northbound.swagger.json

api/proto/gosdn/networkelement/networkelement.proto

api/proto/gosdn/plugin/plugin.proto

api/proto/gosdn/pnd/pnd.proto

applications/arista-routing-engine/network-element.go

applications/hostname-checker/app.go

applications/venv-manager/venv-manager/venv-manager.go

cli/adapter/PndAdapter.go

cli/adapter/PndAdapter_test.go

cli/cmd/networkElementDelete.go

cli/cmd/networkElementSet.go

cli/cmd/networkElementSubscribe.go