Add endpoint that returns current timestamp

[devbuddytest[bot]]

This pull request introduces a new endpoint /timestamp that returns the current timestamp in ISO format. The implementation includes:

• A new router time.py in my_tiny_service/api/routers for handling the timestamp endpoint.
• Modification to my_tiny_service/api/api.py to include the new router in the API.
• A new test case in tests/test_api_timestamp.py to ensure the endpoint behaves as expected.

This feature addresses the need for a simple endpoint that can be used to check the current time in a standardized format.