Resolve "Update gRPC abstraction API with missing calls and refactoring"

Description

Complete:

• User
• Role
• Pnd
  • To dicuss: CreatePnd call, uses CreatePndList, should name of the API call be the same even if we only gice the option to add one? Can we make it optional to add multiple in the API call too?
  • Maybe do only one call where a list of one or many is provided?
• Plugin

• Maybe move GetPluginSchemaTree from MNE to Plugin?

• MNE

• For creation: Maybe do only one call where a list of one or many is provided?
• Need to discuss Update function again!

• Auth

Missing for now. Maybe implement?:

• ConfigurationManagement
• Route (Not implemented because this will probably get reworked)
• Topology (Not implemented because this will probably get reworked)
• App

Related to #359

controller/api/app.go

+package api
+
+import (
+	"context"
+	"time"
+
+	apb "code.fbi.h-da.de/danet/gosdn/api/go/gosdn/app"
+	nbi "code.fbi.h-da.de/danet/gosdn/controller/northbound/client"
+)
+
+// Register checks if the app already exists and if not creates a new one.
+func Register(ctx context.Context, addr, appname, token string) (*apb.AppRegisterResponse, error) {
+	appClient, err := nbi.AppClient(addr, dialOptions...)
+	if err != nil {
+		return nil, err
+	}
+
+	req := &apb.AppRegisterRequest{
+		Timestamp: time.Now().UnixNano(),
+		Appname:   appname,
+		Token:     token,
+	}
+
+	return appClient.Register(ctx, req)
+}
+
+// Deregister deregisters an app.
+func Deregister(ctx context.Context, addr, appname, token string) (*apb.AppDeregisterResponse, error) {
+	appClient, err := nbi.AppClient(addr, dialOptions...)
+	if err != nil {
+		return nil, err
+	}
+
+	req := &apb.AppDeregisterRequest{
+		Timestamp: time.Now().UnixNano(),
+		Appname:   appname,
+	}
+
+	return appClient.Deregister(ctx, req)
+}