Moving the method checking for the subscription extension status to a new helper class

[wjrosa]

Changes proposed in this Pull Request:

This is a simple PR that moves the is_subscriptions_enabled method to a new helper class (WC_Stripe_Subscriptions_Helper). This will allow us to check for the extension status on any non-gateway classes, avoiding the need of using the WC_Stripe_Subscriptions_Utilities_Trait trait.

Testing instructions

Code review. Check if the tests are still passing.

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• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
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