Add Support for USPS Testing Environment Endpoint

[landerop]

Summary

The library currently hardcodes the production USPS API endpoint https://apis.usps.com in src/index.ts:55. To support development and testing workflows, the library should allow developers to optionally use the USPS Testing Environment endpoint https://apis-tem.usps.com

Current Behavior
The USPS class constructor only accepts clientId, clientSecret, and useTitleCase parameters. The baseUrl property is hardcoded as a readonly field:

readonly baseUrl = 'https://apis.usps.com'

This means all API calls (OAuth token generation, address validation, city/state lookup) always target the production environment, even during development and testing.

[landerop]

Hi @alanhett I want to thank you for the time you took to build this library. While using it I've discovered that it does still not have support for the Testing Environment Endpoints so I took the time to fork the repo and create some changes to support this. I've created this PR to handle it. Let me know what you think