feat: Add public API layer (scope protocols, state types)

[bthebladeprimer]

Summary

• Adds scope protocols defining the public API surface for each payment method (card form, Apple Pay, ACH, Klarna, PayPal, QR code, web redirect, form redirect)
• Adds PrimerCheckoutScope as the top-level entry point for CheckoutComponents
• Adds PrimerCardFormState and PrimerApplePayState observable state types
• Adds provider protocols (CardFormProvider, PaymentMethodSelectionProvider, SelectCountryProvider) for SwiftUI integration
• Adds InputFieldConfig, PrimerFieldStyling, PrimerCheckoutTheme, and PrimerEnvironment for UI configuration
• Includes mock interactors and 6 test files covering scope implementations and state management

Context

PR 6 of 15 in the CheckoutComponents feature split. Defines the merchant-facing API that SwiftUI views bind to. Depends on PR 5 (#1634).

Tracker: https://www.notion.so/32fca65dc30e81dabf66f33e2b58c3a1

Test plan

• Verify scope tests pass (DefaultCardFormScopeTests, PrimerCardFormScopeTests, PrimerPaymentMethodScopeTests, DefaultSelectCountryScopeTests)
• Verify state tests pass (PrimerCardFormStateTests, PrimerApplePayStateTests)
• Confirm mock interactors work correctly in test context
• Run: xcodebuild -workspace PrimerSDK.xcworkspace -scheme PrimerSDKTests -destination "platform=iOS Simulator,name=iPhone 16" test

[github-actions]

	Warnings
⚠️	> Pull Request size seems relatively large. If this Pull Request contains multiple changes, please split each into separate PR will helps faster, easier review.

Generated by 🚫 Danger Swift against 6cd9137