Add Support for Shopify Webhooks

[Prateek32177]

We want to add webhook support for Shopify in tern. Shopify sends events for orders, checkouts, payments, products, and more.

Tasks

Webhook Handler

• Implement /src/platforms/shopify.ts.
• Normalize payloads into tern’s event format.

Platform Config

Add platformConfigs.shopify with supported events (orders/create, orders/paid, checkouts/create).

Test Cases

• Add JSON fixtures for 3+ events (order created, order paid, checkout created).
• Write Jest tests in /tests/shopify.test.ts.

Docs

• Update /docs/platforms.md with setup steps.
• Mention secret verification using HMAC SHA256 (Shopify requirement).

Acceptance Criteria

• Shopify handler implemented
• HMAC secret verification supported
• Tests with 3+ event types
• Docs updated

Helpful Links: Shopify Webhooks Docs