WIP Support for generating WC_Subscriptions

[mattallan]

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• Does your code follow the WordPress' coding standards?
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Changes proposed in this Pull Request:

This PR adds support for generating subscriptions via wp wc generate subscriptions x

I've got this PR set to draft as there's a few more features I want to add:

• Add missing support for the --billing-interval arg
• Add missing support the --create-parent-order arg
• Add support for setting a payment method.
• Add support for generating a random number of renewals
• Add support for scheduling next payment dates (not scheduled due to setting the date prop directly)
• Add documentation to the readme

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How to test the changes in this Pull Request:

Support has been added for the following commands:

wp wc generate subscriptions 10
wp wc generate subscriptions 10 --status=cancelled
wp wc generate subscriptions 10 --billing-period=week

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.