RevenueCatUI: fade in Loading view

[aboedo]

This is a proposal, just adding a fade-in effect when first showing the loading view.

I'll get a screen recording of it, but the idea is that since the placeholder view still won't match the actual paywall, and it can lead to perceived "flicker" when it's too fast to really see the placeholder but too slow for it not to show up, maybe the fade makes things a bit smoother.

Still need to try it out more though.

[RevenueCat-Danger-Bot]

	1 Error
🚫	Label the PR using one of the change type labels

Label	Description
breaking	Changes that are breaking
build	Changes that affect the build system
ci	Changes to our CI configuration files and scripts
docs	Documentation only changes
feat	A new feature
fix	A bug fix
perf	A code change that improves performance
RevenueCatUI	A change to the RevenueCatUI library
refactor	A code change that neither fixes a bug nor adds a feature
style	Changes that don't affect the meaning of the code (white-space, formatting, missing semi-colons, etc
test	Adding missing tests or correcting existing tests
next_release	Preparing a new release
dependencies	Updating a dependency
phc_dependencies	Updating purchases-hybrid-common dependency

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