Add whitelist config for masking exclusions (#53)

Add whitelist config for masking exclusions (#53)

Adds masking.whitelist config option to exclude specific text patterns
from PII masking. Useful for preventing false positives on known text
like company names or product identifiers.

- Add whitelist property to MaskingSchema (default: empty array)
- Add filterWhitelistedEntities function to filter detected PII
- Patterns match if detected text is contained in whitelist entry
  or whitelist entry is contained in detected text