Release USPS v3 compatibility

[bensheldon]

USPS has sunset the Webtool APIs that this gem queries. The new USPS v3 API is pretty different.

I'm imagining that:

• this gem will probably need to have several breaking changes to conform with the new API
• the first update to this gem for the new API won't be feature complete

I myself need the new updated API purely for Address verification, so I imagine that driving the development:

https://developer.usps.com/addressesv3

I'll probably also use this opportunity to release a v1.0 too.

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connects to #36, #29

[jtalk22]

Until this gem gets updated, there are open-source USPS v3 SDKs available for other languages that could serve as reference implementations:

• Python: pip install usps-v3 (PyPI) — 67 tests, handles OAuth token caching, all v3 endpoints
• Node.js: npm install usps-v3 (npm) — TypeScript-first, ESM + CJS dual
• PHP: composer require revaddress/usps-v3-php (Packagist)

All MIT licensed: https://github.com/revereveal

The key v3 differences that would affect a Ruby port:

1. OAuth 2.0 client_credentials flow instead of USERID (token valid ~8 hours, needs caching)
2. JSON REST instead of XML — all field names changed to camelCase
3. Rate limits dropped from ~6,000 req/min to 60 req/hour for new apps
4. Separate scopes per endpoint (addresses, tracking, prices, labels)

Endpoint mapping from Web Tools to v3: https://revaddress.com/blog/usps-web-tools-endpoint-mapping