build: add Go module and basic HTTP server using gin-gonic/gin

- Add a new file `go.mod` with the content of a Go module
- Specify the module name as `example`
- Specify the Go version as `1.21.2`
- Add a requirement for the package `github.com/gin-gonic/gin` with version `v1.9.1`
- Add multiple indirect requirements for various packages
- Add a new file `main.go` with a basic Go program using the `gin-gonic/gin` package to create an HTTP server

Signed-off-by: appleboy <appleboy.tw@gmail.com>

main.go

@@ -0,0 +1,20 @@
+package main
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+func main() {
+	r := gin.Default()
+	r.GET("/ping", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{
+			"message": "pong",
+		})
+	})
+	// listen and serve on 0.0.0.0:8080 (for windows "localhost:8080")
+	if err := r.Run(); err != nil {
+		panic(err)
+	}
+}