Add proactive messaging support to AgentApplication

[vrajroutu]

Summary

• introduce a proactive feature area (packages/agents-hosting/src/app/proactive/…) that captures the pair (TurnContext.identity, ConversationReference) on every turn,
  supports manual get/save/delete, honors optional TTLs, and uses storage eTags for optimistic concurrency
• add AgentApplication.proactive, wired through new proactiveOptions, so apps can auto-persist references, retrieve them later, and call sendActivities /
  sendToReference to deliver proactive activity batches while getting back the connector’s activity IDs
• include an optional Express extension (registerProactiveRoutes) that surfaces /api/sendactivity and /api/sendtoreference, enabling external services (e.g., background
  jobs, contact centers) to trigger proactive sends without hand-rolling Bot Framework plumbing
• update the README with configuration and usage guidance, refresh the API baseline to capture the new surface, and cover the feature with targeted unit tests (auto-persist,
  proactive send, TTL expiry, missing-reference error path)

Resolves #336, #414 & #159

[benbrown]

@vrajroutu thank you for this substantial pull request! We have definitely seen this and will review it and get back to you asap. We are in the middle of a release cycle right now so there will be a bit of a delay, but rest assured the team is on it.