Add UnmarshalXML methods that set the internal 'set' flag during XML
parsing. Previously these flags were only set programmatically, causing
XML round-trip to drop elements. This ensures lifecycle configurations
stored as XML survive unmarshal/marshal cycles correctly.
Add comprehensive XML round-trip tests for all lifecycle rule types
including NoncurrentVersionExpiration, AbortIncompleteMultipartUpload,
Filter with Tag/And/size constraints, and a complete Terraform-style
lifecycle configuration.
