Introduces an @InternalRevenueCatAPI annotation

[JayShortway]

In preparation of having to make the Paywalls V2 types public in purchases, so revenuecatui can consume them, this PR adds a new @InternalRevenueCatAPI annotation.

Precedence

There's a lot of precedence for using a @RequiresOptIn annotation to mark a public API as internal. Some examples:

• JetBrains:
  • InternalCoroutinesApi
  • InternalSerializationApi
• Google:
  • InternalComposeApi
  • InternalFoundationApi

Options considered

I experimented a bit with various ways of marking public APIs as internal, and found that using a @RequiresOptIn annotation is the only one that can actually fail compilation. All of the options below can be worked around by either opting in or suppressing them. However, that is a conscious decision by the developer.

	IDE behavior		Compiles?
	Kotlin	Java	Kotlin	Java
@kotlin.RequiresOptIn	red underline	red underline	❌	✅
@androidx.annotation.RestrictTo	red underline	red underline	✅	✅
@androidx.annotation.RequiresOptIn	red underline	red underline	✅	✅

Also in this PR

• Applies the annotation to all new Paywalls V2 types. (Types are not yet public.)
• Opts in module wide in revenuecatui, and for all tests in purchases.

[JayShortway]

Following the Kotlin and Android naming conventions, this would be InternalRevenueCatApi, but I made "API" uppercase for consistency with our existing ExperimentalPreviewRevenueCatPurchasesAPI. Let me know what you think.

[codecov]

Codecov Report

Attention: Patch coverage is 36.17021% with 30 lines in your changes missing coverage. Please review.

Project coverage is 81.89%. Comparing base (7ca10e0) to head (49a7775).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...t/purchases/paywalls/components/ButtonComponent.kt	33.33%	1 Missing and 3 partials ⚠️
...s/paywalls/components/common/ComponentOverrides.kt	0.00%	0 Missing and 3 partials ⚠️
...at/purchases/paywalls/components/ImageComponent.kt	0.00%	0 Missing and 2 partials ⚠️
...at/purchases/paywalls/components/StackComponent.kt	0.00%	0 Missing and 2 partials ⚠️
...cat/purchases/paywalls/components/TextComponent.kt	0.00%	0 Missing and 2 partials ⚠️
...ses/paywalls/components/common/ComponentsConfig.kt	0.00%	0 Missing and 2 partials ⚠️
...rchases/paywalls/components/common/Localization.kt	33.33%	2 Missing ⚠️
...chases/paywalls/components/properties/ColorInfo.kt	33.33%	0 Missing and 2 partials ⚠️
...chases/paywalls/components/properties/ImageUrls.kt	0.00%	0 Missing and 2 partials ⚠️
.../purchases/paywalls/components/PackageComponent.kt	0.00%	0 Missing and 1 partial ⚠️
... and 8 more

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1938   +/-   ##
=======================================
  Coverage   81.89%   81.89%           
=======================================
  Files         260      260           
  Lines        8449     8449           
  Branches     1226     1226           
=======================================
  Hits         6919     6919           
  Misses       1032     1032           
  Partials      498      498           

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