Introduce internal Core runtime plumbing

[heyitsaamir]

Summary

• Adds an internal Core layer for runtime plumbing: HTTP server, cloud/service URL setup, token manager, and authenticated bot/app clients.
• Keeps App focused on app stuff: routing, plugins, context, send/reply, and user OAuth flows.
• Adds an authorize hook for bot/app outbound tokens, while keeping the existing token(scope, tenantId) fallback path.
• Keeps Core out of the root public API. No swappable public Core yet, no drama.

Test plan

• npm --workspace @microsoft/teams.apps run build -- --pretty false
• npm --workspace @microsoft/teams.botbuilder run build -- --pretty false
• npm --workspace @microsoft/teams.apps test -- --runTestsByPath src/app.spec.ts src/token-manager.spec.ts --runInBand