Enable async API client by default when using example app

[mats-stripe]

Summary

Turns on the async API client by default when using the FC Example app. API client used in production is still the "legacy" Futures / Promise based client. This change will have no effect outside of example app usage

Motivation

This for us to dogfood this client to gain confidence in it

Testing

N/a

Changelog

N/a

[github-actions]

⚠️ Public API changes detected:

StripePaymentSheet

- case integrationError(nonPIIDebugDescription: Swift.String)

If you are adding a new public API consider the following:

• Do these APIs need to be public or can they be protected with @_spi(STP)?
• If these APIs need to be public, assess whether they require an API review.

If you are modifying or removing a public API:

• Does this require a breaking version change?
• Do these changes require API review?

If you confirm these APIs need to be added/updated and have undergone necessary review, add the label modifies public API to this PR to acknowledge and bypass this check.

ℹ️ If this comment appears to be left in error, make sure your branch is up-to-date with master.