Fix removeUntaggedEc2: validate billing tags for EKS instances

Previously, EKS clusters without billing tags were deleted if they didn't
match the skip pattern, even if they had valid billing tags. This fix adds
proper billing tag validation for EKS instances.

Changes:
- Add has_valid_billing_tag() to validate category strings and timestamps
- Check billing tags BEFORE skip pattern for EKS instances
- Support Unix timestamp expiration in iit-billing-tag
- Add comprehensive test suite with 12 test scenarios
- All tests pass with proper timezone handling (UTC)

Author: nogueiraanderson