Release 0.4.0

[AltiAntonov]

Summary

HTTP cache-validation release.

Changes

• persist HTTP validation metadata with cached snapshots
• add conditional request header support for If-None-Match and If-Modified-Since
• capture response validation metadata from the built-in HTTP fetcher
• detect 304 Not Modified responses explicitly
• renew cached snapshot freshness when HTTP revalidation returns 304
• add an HTTP Cache Validation example scenario in the showcase app

Example App Impact

• adds a dedicated scenario entry for HTTP cache validation
• shows server ETag, cached ETag, and last response type
• demonstrates 200 OK vs 304 Not Modified revalidation flow

Documentation Impact

• no public docs or changelog updates yet in this branch
• private roadmap state has been updated outside the package repo

Verification

• swift test
• xcodebuild -scheme RemoteConfigStoreExample -project /Users/aantonov/Developer/Own/Packages/RemoteConfigStore/Example/RemoteConfigStore/RemoteConfigStore.xcodeproj -destination 'generic/platform=iOS Simulator' build

Release Impact

• adds the core HTTP cache validation path needed for 0.4.0
• keeps optional diff-aware update handling and configurable refresh interval strategy out of this PR

Checklist

• package tests pass
• example app builds
• branch pushed to origin
• changelog updated
• README / DocC release polish if needed