Adds cardholder_name attribute reader for Apple Pay Card

[AxelTheGerman]

Summary

See https://github.com/braintree/braintree_ruby/issues/227 which describes the issue of cardholder_name instance variable being assigned but not exposed via attr_reader unlike the regular CreditCard object.

Checklist

• [x] Added changelog entry (If there isn't an #unreleased section, add that and your changelog entry to the top of the changelog)
• [x] Ran unit tests (rake test:unit)
• [x] I understand that unless this is a Draft PR or has a DO NOT MERGE label, this PR is considered to be in a deploy ready state and can be deployed if merged to main

[hollabaq86]

for internal tracking, ticket 1717

[Dani-Kirby]

@AxelTheGerman Thank you for submitting this PR! Changes will be added in the next release.