gma_mediation_meta: add SwiftPM (SPM) support — extends #1395 to the Meta mediation package

[qeepcologne]

Summary

#1395 added Swift Package Manager support to the core google_mobile_ads plugin, but the mediation packages weren't included. This PR extends the same treatment to gma_mediation_meta, so apps using Flutter's SwiftPM iOS integration (no CocoaPods) can link the Meta Audience Network mediation adapter.

• Adds packages/mediation/gma_mediation_meta/ios/gma_mediation_meta/Package.swift. The SPM target depends on MetaAdapterTarget from Google's googleads-mobile-ios-mediation-meta package — the SPM equivalent of the GoogleMobileAdsMediationFacebook pod the podspec declares.
• Mirrors the dual podspec + SPM layout Adds ‎google_mobile_ads support for Swift Package Manager #1395 established for google_mobile_ads: sources moved from ios/Classes/ to ios/gma_mediation_meta/Sources/gma_mediation_meta/, podspec source_files repointed — both CocoaPods and SPM consumers keep working.

Context

Distinct from #1395 (that one is the core plugin; this is the Meta mediation adapter package). The same approach would apply to the other gma_mediation_* packages.

Test plan

• CocoaPods consumer still resolves (pod lib lint).
• SPM consumer resolves MetaAdapterTarget and links the Meta adapter.

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[qeepcologne]

Closing — SwiftPM rollout for this repo is being handled upstream (core landed in #1395). Keeping these mediation-package changes in our fork for now.