Carrier for USPS

Initial release.

Added

• USPS APIs v3 carrier for Carrier: OAuth client credentials plus a cached label payment authorization token (CRID, MID, EPS or permit account).
• Live rates from Shipping Options: every class in one call per distinct package, single-piece counter prices only, content-restricted classes kept out of rate shopping.
• Domestic, international (with customs) and return labels as PDF 4×6, ZPL or PNG, using the JSON label response rather than multipart.
• Multi-piece shipments, one label per package, with an honest partial-failure policy: bought labels are voided on a refusal, and anything doubtful is parked as uncertain.
• Voids (cancel or refund dispute), reprints, Tracking 3.2 batch tracking, carrier pickups, US address validation.
• Post Office search and Hold for Pickup services for Ground Advantage, Priority Mail and Priority Mail Express.
• Connection test that reports which USPS APIs the app has been granted and whether the payment account can pay for labels.