[6.x] Reference gateways & shipping methods by handles

[duncanmcclean]

This pull request makes some necessary changes to the way Gateways & Shipping Methods are stored in order data.

Previously, the fully qualified class name was saved to the order's YAML:

shipping_method: DoubleThreeDigital\SimpleCommerce\Shipping\FreeShipping

However, with this PR, gateways & shipping methods are now references by their "handles":

shipping_method: free_shipping

References in orders will be automatically updated upon updating.

These changes were necessary as part of the change in namespace from DoubleThreeDigital to DuncanMcClean which is due to happen in the v6 release.

This PR is a draft for now since there's a couple of rough edges I want to iron out before merging.

To Do

• Consider completely deprecating the class-based syntax to reduce tech debt & metal overhead.
  • Only accept handles in "form" requests (search TripleFourDigital\\ComplexCommerce\\SmellyGatewayHaha as reference)
• Move the code from UpdateClassReferences into a command so it can be run manually if needed.
• Add notes to upgrade guide
  • "The way gateways / shipping methods are references in orders has changed"
  • Handles for gateways are now generated differently. You should review your payment page & any other integrations you have which may rely on the existing handles.
• Ensure the payment page in the Starter Kit is up-to-date (do references to hard-coded class names need changing to handles?)
• Fix failing tests

[github-actions]

Released as part of v6.0.0.