Add time_tools example (now, to_seconds, add_seconds)

[NA-V10]

Summary

This PR adds a new example module time_tools to the examples/ directory. It follows the same format and structure as the existing Google examples such as cipher, including licensing headers, function layout, Fire usage style, and test patterns.

Details

• New folder: examples/time_tools/
• Provides simple time utility functions:
  • now() — returns the current datetime string
  • to_seconds(minutes, seconds) — converts to total seconds
  • add_seconds(time_string, seconds) — adds seconds to a datetime
• Fire CLI is invoked without a target component, so all module functions become CLI commands, consistent with other examples.
• Includes a full test suite built using testutils.BaseTestCase.

Checklist

• [x] Example follows Google style (license header, naming, docstrings)
• [x] Test file included following Google's cipher_test.py pattern
• [x] No modifications to core Fire library
• [x] CLA signed