Add uiPreviewMode to DangerousSettings

[ajpallares]

Checklist

• If applicable, unit tests
• If applicable, create follow-up issues for purchases-android and hybrids

Motivation

We need a way to activate the UI Preview mode in the SDK to enable the Paywall Previews functionality in the RC app without actually adding any new public APIs to the SDK

Description

This PR adds a new property to DangerousSettings: uiPreviewMode.

• This new public API is opt-in only, thanks to the @_spi attribute.
• The UI Preview Mode of the SDK will limit all the functionality of the SDK to only enable implementing the Paywall Previews functionality (and similar) in the RevenueCat app, so that developers can properly test their paywalls on their own devices.

[ajpallares]

I have added the uiPreviewMode as a DangerousSettings:

1. This should make it double clear that it's not for public use (DangerousSettings + @_spi attribute). In fact, the @_spi attribute does prevent the use of this API for regular imports of the SDK.
2. In a future major release, we can move everything that currently lives under DangerousSettings, to use the @_spi attribute. So we can use this uiPreviewMode as a first step towards that goal.

Of course, I'm open to suggestions about adding the uiPreviewMode to a better-fitting place.

[aboedo]

I triggered a re-run of the iOS 18 tests since the failures seemed unrelated

[aboedo]

awesome!

[aboedo]

maybe we can add the API tests to go along with it in this same PR? Those serve to also showcase the usage

[ajpallares]

Done in f406e82

[JayShortway]

@_spi all the things! 🙌

I don't have a strong opinion on whether it should be in DangerousSettings or not. I'd also be okay to just move this to the regular Configuration. However, since it's internal we can always change this later.

[ajpallares]

@RCGitBot please test

[tonidero]

LGTM! Nice knowing this 💪