feat: add test-only native ads research preview

[rdlabo]

Summary

• add a framework-independent NativeAdFeed API and custom element for stable native-ad slots
• render plugin-owned Google Mobile Ads native views on Android and iOS
• implement the new Android feature in Kotlin while preserving the existing Java plugin boundary
• add feed sessions, placement sequencing, lifecycle barriers, size validation, and bounded resource usage
• add a dedicated Ionic/Angular demo tab with stable feed slots, test-ad reloads, event output, and lifecycle cleanup
• document Angular, React, and Vue integration experiments without requiring app-side Kotlin or Swift

Research preview boundaries

• this is an unsupported, test-ads-only research preview and is not listed as a production feature
• TypeScript and both native implementations force Google's official native test ad units
• browsers and PWAs reject feed startup instead of silently rendering an empty slot
• the demo uses standard Ionic scrolling and explicitly does not claim virtual-scroll support
• production support remains gated on real-device gesture routing, virtual-scroller, lifecycle, accessibility, and Native Ad Validator acceptance tests

Related to #110 and Cap-go/capacitor-admob#1.

Validation

• npm run build
• npm run eslint
• npm run prettier -- --check
• SwiftLint: 0 violations on the affected Swift sources and tests
• Angular demo: 23 tests, lint, and production build
• Android: Kotlin/Java 21 clean build, unit tests, and Android Lint
• iOS: generic-device build and simulator test-bundle build
• npm pack --dry-run

[rdlabo]

/beta

[github-actions]

npm beta published

CI passed and a repository owner or maintainer requested /beta for commit bf4622e18a79. Install the immutable version with:

npm install @capacitor-community/admob@8.1.0-beta.pr454.shabf4622e18a79