feat: add ForwardClient and integrate with CheckoutApi

[armando-rodriguez-cko]

This pull request introduces a new feature to the Checkout SDK: the Forward module, which facilitates forwarding requests to external destinations. The changes include adding the ForwardClient class, integrating it into the SDK's API, defining OAuth scopes for the feature, and adding integration tests. Below are the most important changes grouped by theme:

Feature Implementation:

• Added a new ForwardClient class in lib/checkout_sdk/forward/forward_client.rb, which provides methods for sending forward requests (forward_request) and retrieving them by ID (get).
• Introduced the Forward module in lib/checkout_sdk/forward/forward.rb, which requires the ForwardClient.

SDK Integration:

• Updated lib/checkout_sdk.rb to require the new Forward module.
• Modified the CheckoutApi class in lib/checkout_sdk/checkout_api.rb to include the forward attribute and initialize it with the ForwardClient. [1] [2] [3]

OAuth Scopes:

• Added a new OAuth scope FORWARD to the OAuthScopes module in lib/checkout_sdk/oauth_scopes.rb.

Integration Tests:

• Added integration tests for the Forward module in spec/checkout_sdk/forward/forward_integration_spec.rb. These tests verify the functionality of forward_request and get methods.