STPFormView and StackViewWithSeparator better SwiftUI support

[wooj-stripe]

Summary

Surrouding higher priority views around STPCardFormView was causing the view to completely collapse depsite already setting contentCompressionResistancePriority. To remedy, determine the intrinsic content size such that swift ui can lay this view out better.

Motivation

Adding Spacers() w/ higher priority around this view can cause this view to completely collapse.

Testing

Manual testing

Before	after

Changelog

[wooj-stripe]

Seems that the spacing is off for a few. I will try and match this up the best i can, and there are some places that I've obviously broken... From a quick review:

• Not sure what's going on with: testEmptyHiddenPostalCode
• In testSingleSectionWithTItle -- the title is missing.

[github-actions]

⚠️ Public API changes detected:

StripePaymentsUI

- public var intrinsicContentSize: CoreFoundation.CGSize {
+ @objc get
+ }

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.

[wooj-stripe]

#4284