v0.20.0

Added

• Added a Context::redact_field_mut builder method to tweak metadata
  redaction policies in place before attaching additional fields.
• Extended response tests to cover JSON serialization of redacted payloads and
  hashed metadata along with checks for the opt-in internal formatters.

Changed

• Verified ErrorResponse and ProblemJson serialization respects message and
  metadata redaction policies, ensuring secrets stay out of wire payloads while
  keeping diagnostic logging intact.