Add Contact Module setting to new settings UI (4799)

[stracker-phil]

Description

Added a new toggle option to the "Settings" tab in the new UI admin page.

• The toggle is gated by a new feature flag
• When the feature flag is active, the toggle appears in the "PayPal Settings" accordion
• Default value is currently set to false/off
• No logic is attached to the toggle - it's saved to the DB without doing anything else

Feature flag

How to activate the feature flag:

• Set env variable PCP_CONTACT_MODULE_ENABLED to "1"
• Return true via WP filter woocommerce.feature-flags.woocommerce_paypal_payments.contact_module_enabled

Steps to Test

1. Activate the feature flag / ensure to use the new UI
2. Go to "WooCommerce → Settings → Payments → PayPal"
3. Navigate to tab "Settings", open the accordion "PayPal Settings"
4. The new toggle appears below the toggle "Instant payments only"

Screenshots