Allow dynamic property

Fix deprecated error Allow dynamic properties for PHP 8.3 compatibility

1.0.1

• Remove rgblank function dependency (Gravity Forms is not required)

1.0.0

• Adds a checkbox to subscribe to newsletter on checkout page
• Syncs data only if checkbox is checked by user
• Checkbox is unchecked by default
• If checkbox is unchecked, does not unsubscribe the user if the user is already subscribed
• Sync the billing fields: billing_email, billing_first_name, billing_last_name
• Compatibility with TMSM WooCommerce Billing Fields (billing_birthday, billing_title fields)
• Select the optin field the users will subscribe