Implement methods to synchronize

This is ready for a first review, including the issue for the configuration https://www.drupal.org/project/invoice_ninja/issues/3412582 📌 Implement config settings page Active .

But please consider that the exception handling https://www.drupal.org/project/invoice_ninja/issues/3416633 📌 Exception Handling Active
and the handling of multiple contacts https://www.drupal.org/project/invoice_ninja/issues/3416659 ✨ Enable multiple contacts for clients Active are in progress and in other issues.

Tests are an open task as well.

I've reviewed the code for this issue and it looks pretty good. I've made some changes though, you can see them in the commit. Here are some comments:

• Constants should have an access control identifier
• The pattern around getInvoiceNinjaClient() is such that instead of using $this->invoiceNinja we're using $this->getInvoiceNinjaClient() which sets and returns the initialized object
• For the remote update calls, we don't need to receive the object for the remote ID as we already know that ID that doesn't change during an update
• For updating invoices, there was a copy&paste issue that mixed up client and invoice IDs

Leaving the issue at NR, please feel free to revert my changes when needed.

Thx. That is really cool.

Automatically closed - issue fixed for 2 weeks with no activity.