Add types for session.surcharge and session.surcharge for automatic surcharge support

[sophiaw-stripe]

Summary & motivation

https://jira.corp.stripe.com/browse/OCS_API-6944

Adds the following changes to stripe.js types

export type StripeCheckoutSurchargeStatus = 
 | {status: 'complete'} 
 | {status: 'requires_input'} 
 | {status: 'failed'}

export interface StripeCheckoutSession {
  //... existing fields
  surcharge: StripeCheckoutSurchargeStatus | null;
}

export interface StripeCheckoutTotalSummary {
  // ... existing fields
  surcharge?: StripeCheckoutAmount;
}

As specified in API Review: automatic surcharge via 3rd party in Checkout payment mode

Testing & documentation

API reference updated in https://git.corp.stripe.com/stripe-internal/mint/pull/2072037 , behind gate

[sophiaw-stripe]

r? @pololi-stripe

Do these changes need to be gated or tested?

Did not see prior art for gating, and differing standard for tests
1f67c84
63f58f6

This seems maybe like something we wouldn't maintain a unit test for since it's a nullable + (currently) preview feature?