Add CLI to interact with Diun through gRPC (#382)

Add simple CLI to interact with Diun through gRPC
Create image and notif proto services
Compile and validate protos through a dedicated Dockerfile and bake target
Implement proto definitions
Move server as `serve` command
New commands `image` and `notif`
Refactor command line usage doc
Better CLI error handling
Tools build constraint to manage tools deps through go modules
Add upgrade notes

Co-authored-by: CrazyMax <crazy-max@users.noreply.github.com>

cmd/notif.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/crazy-max/diun/v4/pb"
+)
+
+// NotifCmd holds notif command
+type NotifCmd struct {
+	Test NotifTestCmd `kong:"cmd,help='Test notification settings.'"`
+}
+
+// NotifTestCmd holds notif test command
+type NotifTestCmd struct {
+	CliGlobals
+}
+
+func (s *NotifTestCmd) Run(ctx *Context) error {
+	defer s.conn.Close()
+
+	nt, err := s.notifSvc.NotifTest(context.Background(), &pb.NotifTestRequest{})
+	if err != nil {
+		return err
+	}
+
+	fmt.Println(nt.Message)
+	return nil
+}