Typing for Direct Line objects

Feature Request

As we start to add typings to all of our code, we specifically didn't type Direct Line objects because we didn't think out the plan to correctly type these objects:

1. Import typings from DirectLineJS
   • It's outdated
2. Import typings from botframework-schema
   • It depends on botbuilder-stdlib, which is not a small library
   • Some undocumented code is using botbuilder-stdlib and they are related to assertions
   • We could import { IActivity } from 'botframework-schema/lib/activityInterfaces' but seems a bit hacky
   • We could reach SDK team for a further discussion
3. Write our owns
   • Bigger effort
   • We can fully understand what fields we used
   • When we move to ACS later, understanding these fields helps
   • When we move to ACS, we could consider having our own schema, bringing best of both world

[feature-request]

Start by reviewing the three typing approaches listed in the issue: DirectLineJS, botframework-schema, and writing project-owned types. The issue does not name files, tests, or an entry point, and completion is not defined beyond selecting and implementing a typing strategy for Direct Line objects, so clarification is needed before starting.