Add Endpoint for Current Timestamp in ISO Format

[acebots-ai[bot]]

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

• Adding the get_current_timestamp function in my_tiny_service/api/routers/root.py.
• Importing datetime in my_tiny_service/api/routers/root.py for timestamp generation.
• Adding a test case test_get_current_timestamp in tests/test_api.py to ensure the endpoint's functionality.

This feature enhances the service by providing a simple way to fetch the current time in a standardized format.