feat: add example for Cron and Target (#715)

## Example 10: Target property and Cron feature

### Steps to run the example

#### 1. Start Zipper Server

```bash
yomo serve -c ../config.yaml
```

#### 2. Start `sfn-1-executor`

This stateful serverless function will emit data every 2 seconds.

```bash
go run sfn-1-executor/main.go
```

#### 3. Start two instances of `sfn-2-sink` to consume the data

First one start with `USERID=alice`, this instance will consume the data
with `target` property set to `alice`.

```bash
USERID=alice go run sfn-2-sink/main.go
```

Second one start with `USERID=bob`, this instance will consume the data
with `target` property set to `bob`.

```bash
USERID=bob go run sfn-2-sink/main.go
```

<img width="1912" alt="image"
src="https://github.com/yomorun/yomo/assets/65603/e220b06e-3030-46c9-ad11-51382772006d">

example/a-target-and-cron/README.md

@@ -0,0 +1,31 @@
+## Example 10: Target property and Cron feature
+
+### Steps to run the example
+
+#### 1. Start Zipper Server
+
+```bash
+yomo serve -c ../config.yaml
+```
+
+#### 2. Start `sfn-1-executor`
+
+This stateful serverless function will emit data every 2 seconds.
+
+```bash
+go run sfn-1-executor/main.go
+```
+
+#### 3. Start two instances of `sfn-2-sink` to consume the data
+
+First one start with `USERID=alice`, this instance will consume the data with `target` property set to `alice`.
+
+```bash
+USERID=alice go run sfn-2-sink/main.go
+```
+
+Second one start with `USERID=bob`, this instance will consume the data with `target` property set to `bob`.
+
+```bash
+USERID=bob go run sfn-2-sink/main.go
+```

example/a-target-and-cron/sfn-1-executor/main.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"fmt"
+	"log/slog"
+	"os"
+
+	"github.com/yomorun/yomo"
+	"github.com/yomorun/yomo/serverless"
+)
+
+var i = 0
+var target = "bob"
+
+func main() {
+	sfn := yomo.NewStreamFunction(
+		"fn1",
+		"localhost:9000",
+	)
+	defer sfn.Close()
+
+	sfn.SetCronHandler("@every 1s", func(ctx serverless.CronContext) {
+		if i%2 == 0 {
+			target = "alice"
+		} else {
+			target = "bob"
+		}
+		// ctx.Write(0x33, []byte("message from cron sfn"))
+		ctx.WriteWithTarget(0x33, []byte(fmt.Sprintf("message from cron sfn %d", i)), target)
+		i++
+	})
+	// start
+	err := sfn.Connect()
+	if err != nil {
+		slog.Error("[sfn] connect", "err", err)
+		os.Exit(1)
+	}
+	// set the error handler function when server error occurs
+	sfn.SetErrorHandler(func(err error) {
+		slog.Error("[sfn] receive server error", "err", err)
+	})
+
+	sfn.Wait()
+}

example/a-target-and-cron/sfn-2-sink/main.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"os"
+
+	"github.com/yomorun/yomo"
+	"github.com/yomorun/yomo/serverless"
+	"golang.org/x/exp/slog"
+)
+
+var instanceID = "bob"
+
+func main() {
+	if v := os.Getenv("USERID"); v != "" {
+		instanceID = v
+	}
+	sfn := yomo.NewStreamFunction(
+		"sink",
+		"localhost:9000",
+	)
+	sfn.SetObserveDataTags(0x33)
+	sfn.SetWantedTarget(instanceID)
+
+	// set handler
+	sfn.SetHandler(handler)
+	// start
+	err := sfn.Connect()
+	if err != nil {
+		slog.Error("[sfn] connect", "err", err)
+		os.Exit(1)
+	}
+	defer sfn.Close()
+
+	// set the error handler function when server error occurs
+	sfn.SetErrorHandler(func(err error) {
+		slog.Error("[sfn] receive server error", "err", err)
+	})
+
+	sfn.Wait()
+}
+
+func handler(ctx serverless.Context) {
+	data := string(ctx.Data())
+	slog.Info("Received", "uid", instanceID, "tag", ctx.Tag(), "data", data)
+}