chore: remove deprecated, unused add_credit_card function

[Justin Hammond (Justintime50)]

Description

• Removes the deprecated, unusable add_credit_card function
  • Stripe has disabled the ability to pass plain credit card details over the wire and now requires using Stripe.js/Elements/Checkout. Follow the Decentralized (EasyPost-Manage Billing) Guide for more details on the new flow to use.
  • Makes referral_customer.retrieve_easypost_stripe_api_key public to help facilitate adding credit cards using Stripe.js

Also removes the build dev dep for the builtin uv build command to reduce complexity

Testing

Adds a new test for the public function, removes the old tests for the removed function

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)