Upgrade to Recurly PHP v3 library

The MR updates the module to work with the latest release of the Recurly PHP SDK. This required making a lot of updates to the code since the way the Recurly PHP classes are used is different in the current version than the older v2 SDK the module has been using. Hence the need to touch so many different files. I've tried to stick to only updating the API integration and not changing any of the modules functionality. I've verified this work so far through a combination of manual testing, the tests included in the module, and via Drupalize.Me's e2e testing suite.

This update is going to have breaking changes so I think we should create a new 6.x branch, and start with a 6.x-beta or something like that that indicates I think this is working (at least in my tests) but you should also do your due diligence if upgrading. We could also have this new branch drop support for Drupal 9.

@eojthebrave thx for your work on this! I would love to test this but with both the 5.x branch and this patched version I'm not able to load active subscriptions for a user on the /user/nid/subscription page. It will always decide there are no subscriptions for the user and redirect to /user/uid/subscription/signup.
I am able to load the list of plans at /admin/config/services/recurly/subscription-plans so my API keys should be fine.

Is this a known problem on Drupal 10.x and PHP 8.3? Need a workaround for this or help debugging it in order to help you test your patch. Unfortunately I don't get any errors and to debug the API request I think I'd need to edit the recurly-client library, or is there a possibility to get debug info my flipping some verbosity switch?

Even when I create a new susbcription from within the Drupal site, the new subscription will be created with the correct account code (user-uid) but not load for said user.