feat: add Features server/client package with Vapor example

[AdonisCodes]

Summary

• add FeaturesServer with deterministic bucketing, request-time evaluation middleware, latest-only override audit rows, and auth-protected routes
• add FeaturesClient with fetch/toggle APIs
• scaffold FeaturesShared and FeaturesAdmin targets
• add runnable Vapor + Fluent SQLite example app and simple client executable

Notes

• FeaturesServer.configure registers migrations and feature routes
• overrides are append-only audit rows with latest=true semantics
• no tests included yet (as requested)