chore: remove deprecated, unused AddCreditCardToUser function

[Justin Hammond (Justintime50)]

Description

• Removes the deprecated, unusable ReferralCustomer.AddCreditCardToUser 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 ReferralCustomer.RetrieveEasypostStripeApiKey public to help facilitate adding credit cards using Stripe.js

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)