Add time.Duration support for cache expiration (Issue #104)

[DARSHANR007]

Implements time.Duration as a first-class API for cache expiration settings.

Changes:

• Added 5 new Duration-based methods: SetDuration(), TouchDuration(), GetOrSetDuration(), SetAndGetDuration(), UpdateDuration()
• Added UpdaterDuration callback type
• Marked old int-based methods as Deprecated (they now delegate to Duration methods)
• Comprehensive unit tests for all new methods
• All 41 tests pass, full backward compatibility maintained

Closes #104