Adding a new feature flag and setting to control SPE

[wjrosa]

Fixes #3892

Changes proposed in this Pull Request:

This PR introduces a new feature flag (_wcstripe_feature_spe) to control the Single Payment Element availability. This flag controls the feature as a whole, and does not behave like an option.

When the flag is enabled, and the legacy checkout experience is disabled, you should be able to toggle the "Single payment element" feature in settings. When the flag is disabled, the option is not displayed.

Testing instructions

• Checkout and build this branch on your test environment (add/spe-feature-flag)
• Connect your Stripe account
• Enable the new feature flag (_wcstripe_feature_spe). You can do it by either hardcoding the return value of is_spe_available to true or by running npm run wp option update _wcstripe_feature_spe 'yes'
• As a merchant, disable the legacy checkout experience in settings (wp-admin/admin.php?page=wc-settings&tab=checkout&section=stripe&panel=settings)
• Confirm that you can see the new "Single payment element" setting:

• Confirm that you can toggle it and that the value you set persists upon page refresh
• Disable the _wcstripe_feature_spe feature flag
• Confirm that the option is gone even when the legacy checkout experience is disabled

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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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• 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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[Mayisha]

@wjrosa thanks for adding this. The changes are mostly working except the saving part.

✅ SPE checkbox is displayed when the feature flag is enabled.
✅ SPE checkbox is hidden when the feature flag is disabled.
✅ SPE checkbox is disabled when legacy is enabled.
⚠️ Enabling the SPE checkbox does not persist on page refresh.