[WIP] Coalesce MPE update calls when sheet is open

[davidme-stripe]

Summary

Motivation

Testing

Changelog

[github-actions]

⚠️ Public API changes detected:

StripePaymentSheet

- public var cornerRadius: CoreFoundation.CGFloat?
- public var cornerRadius: CoreFoundation.CGFloat

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.

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