Enable "Stripe key swapper"

[mircobabini]

All Submissions:

• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

After some discussion on this topic, the best idea we came up with was adding 3 hooks.

The idea is to permit an additional snippet to change the api key Stripe is using when dealing with:

• webhook requests (by checking an additional query arg passed by the webhook setup itself)
• member orders (by checking the date of the first order of the chain)
• subscriptions (by checking the sub start date)

Then, use the swap key call accordingly: Stripe\Stripe::setApiKey( 'key_to_use' )

Discussion on the slack channel: https://paidmembershipspro.slack.com/archives/GSVNCKRUG/p1712828786645169

Other information:

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