Add json + interfaces exercise

json/README.md

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+Source guide: https://jordanorelli.com/post/32665860244/how-to-use-interfaces-in-go

json/go.mod

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+module json
+
+go 1.22.2

json/main.go

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+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"reflect"
+	"time"
+)
+
+type Timestamp time.Time
+
+func (t *Timestamp) UnmarshalJSON(b []byte) error {
+	v, err := time.Parse(time.RubyDate, string(b[1:len(b)-1]))
+	if err != nil {
+		return err
+	}
+	*t = Timestamp(v)
+	return nil
+}
+
+// start with a string representation of our JSON data
+var input = `
+{
+    "created_at": "Thu May 31 00:00:01 +0000 2012"
+}
+`
+
+func main() {
+	// our target will be of type map[string]interface{}, which is a
+	// pretty generic type that will give us a hashtable whose keys
+	// are strings, and whose values are of type interface{}
+	var val map[string]Timestamp
+
+	if err := json.Unmarshal([]byte(input), &val); err != nil {
+		panic(err)
+	}
+
+	fmt.Println(val)
+	for k, v := range val {
+		fmt.Println(k, reflect.TypeOf(v))
+	}
+
+	fmt.Println(time.Time(val["created_at"]))
+}