Add Vipps PaymentSheet support and preview betas

[fionnbarrett-stripe]

Summary

• Add Vipps as a MobilePay-parity PaymentSheet payment method, including type wiring, PaymentSheet definition/registry updates, strings, assets, and tests.
• Add merchant-owned beta plumbing so vipps_preview=v1 can flow through PaymentConfiguration, ApiRequest.Options, and request headers without SDK auto-injection.
• Propagate betas through PaymentSheet, checkout-session, redirect result handling, customer/session repositories, and status polling.
• Validated with targeted compile and unit-test runs locally.
• Plan and API review notes: https://git.corp.stripe.com/gist/941a3dc8c76815a377bbd70bdf8e18c0

Motivation

Vipps should behave like MobilePay on Android unless code proves otherwise, but it is still gated behind a private preview header that merchants must opt into themselves. This wires the Android SDK for PaymentSheet support while keeping header ownership with the merchant-owned client configuration.

Testing

• Added tests
• Modified tests
• Manually verified

Screenshots

Before	After
N/A	N/A

Changelog

No changelog entry. This is PaymentSheet support for a private-preview payment method and request-header plumbing.