feat: ad tracking API

[peterporfy]

Summary

Adds ad impression and revenue tracking support to the Unity SDK.

Design notes

• All types in namespace RevenueCat — no extra using needed beyond using RevenueCat;
• purchases.AdTracker returns a RevenueCat.AdTracker instance (initialized in Purchases.Start())
• Value types nested in AdTracker (AdTracker.Format, AdTracker.MediatorName, AdTracker.Precision) to avoid naming collisions with ad SDK types like GoogleMobileAds.Api.AdFormat. This is different compared to the rest of the SDKs as C# using directives do wildcard namespace injection and that results in an immediate AdFormat clash between RC and Google. Since we are nesting under AdTracker I dropped the Ad prefix from the Format, Mediator etc types.
• Data is passed as JSON strings across the C#/native boundary, consistent with existing SDK patterns
• Optional string fields (networkName, placement) are omitted from JSON when null: SimpleJSON's Escape() crashes on null strings, so null checks are required at serialization time
• mediatorErrorCode (int?) follows the same pattern: key omitted when null

Other notes

• I am using simple remarks (https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/xmldoc/recommended-tags#remarks) to signal the API is still experimental. I didn't find anything stronger. I considered https://learn.microsoft.com/en-us/dotnet/api/system.obsoleteattribute?view=netframework-4.8.1&viewFallbackFrom=net-10.0 - but it is semantically incorrect, and https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-12.0/experimental-attribute - but it is not supported in older cs.

Tests

• added API surface tests
• extended the sample app locally and manually tested builds both with android and ios target