Introducing a Stripe order helper class

[wjrosa]

See peNR48-1Jc-p2

Changes proposed in this Pull Request:

This PR introduces a new helper class to handle order-specific data within the extension. This is related to the previous attempt to add an order model class, which we reverted due to incompatibility.

Instead of a model, this new class will retrieve and update WC_Order data (from existing instances), so we should not encounter instantiation issues.

In this first part, I am adding some querying methods and some flags. In the second part, I will add most of the getters and setters.

Testing instructions

• Code review
• Check if the tests are still passing
• Perform some basic smoke testing to confirm no regression was introduced

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• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

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