feat: add time zone query handling

[carlhus]

Description

This pull request introduces a Time Zone API in the nf-example project, supporting the following operations:

• Query the time zone of a city via GET /timezone/city/:City
  > curl -X GET http://127.0.0.163:8000/timezone/city/Taipei -w "\n"

• Add a new city and its time zone via POST /timezone/city with JSON payload
  > curl -X POST http://127.0.0.163:8000/timezone/city -d '{"City": "Chicago", "TimeZone": "UTC-5"}' -w "\n"

• Reset an existing city’s time zone via POST /timezone/city/:City with JSON payload
  > curl -X POST http://127.0.0.163:8000/timezone/city/Chicago -d '{"TimeZone": "UTC-6"}' -w "\n"

• Delete a city’s time zone entry via DELETE /timezone/city/:City
  > curl -X DELETE http://127.0.0.163:8000/timezone/city/Chicago -w "\n"