feat: Implement flag update

[inourbubble2]

Related to #41

request:

curl -X 'PUT' \
  'http://localhost:8080/api/v1/flags/user-limit' \
  -H 'accept: */*' \
  -H 'Authorization: Bearer eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIyIiwidG9rZW5UeXBlIjoiYWNjZXNzIiwidXNlciI6eyJ1c2VyTmFtZSI6InN0cmluZyIsInVzZXJJZCI6IjIiLCJsYXQiOiIyMDI1LTAyLTI4VDE0OjA2OjM4LjY5NDkwMSJ9LCJpYXQiOjE3NDA3MTkxOTgsImV4cCI6MTc0MDcyMDk5OH0.gZJIRSTmKMG_VqG6qZhngQf9d2l2WWSmEhK6wdgQg1A' \
  -H 'Content-Type: application/json' \
  -d '{
  "key": "user-limit2",
  "type": "number",
  "defaultValue": {
    "number": 100
  },
  "description": "description updated",
  "variants": [
    {
      "free": 1000
    }
  ]
}'

response:

{
  "status": "Success",
  "message": "Updated flag successfully",
  "data": {
    "key": "user-limit2",
    "status": true,
    "type": "NUMBER",
    "defaultValue": {
      "number": 100
    },
    "description": "description updated",
    "variants": [
      {
        "number": 100
      },
      {
        "free": 1000
      }
    ],
    "createdBy": "username",
    "updatedBy": "string",
    "createdAt": "2025-02-17T00:27:06.122Z",
    "updatedAt": "2025-02-28T05:06:50.389395Z"
  }
}