Create ActivityHandlerBase, rewrite ActivityHandler for increased extensibility points

[stevengum]

Description

This PR adds the ActivityHandlerBase with new on*Activity() methods. These methods mirror the corresponding C# activities.

It also refactors the core logic in ActivityHandler and ActivityHandler.run() out into separate overrideable methods. This increases extensibility as developers can change the processing logic and reuse the base logic.

The original eventing logic is still included in ActivityHandler.

None of the already public APIs in ActivityHandler have been modified, and ActivityHandler.run() is still the only point of ingesting activities.

Specific Changes

• New ActivityHandlerBase class used for subclassing. (Closely follows the C# impl of ActivityHandler)
  • New tests for ActivityHandlerBase
• ActivityHandler extends ActivityHandlerBase and overrides the inherited on*Activity() methods from ActivityHandlerBase.
  • The overrides still emit the appropriate events.
• ActivityHandler now has protected methods that dispatch specific activity types by additional properties on the incoming Activity.
  • dispatchConversationUpdateActivity (called by overridden onConversationUpdateActivity)
  • dispatchMessageReactionActivity (called by overridden onMessageReactionActivity)
  • dispatchEventActivity (called by overridden onEventActivity)

Testing

Current ActivityHandler unit tests still work, will add new unit tests (and maybe a TypeScript test collection 🚀)

Todo:

• Unit tests for refactored logic
• Unit tests for subclassing ActivtityHandler (Possibly also include TypeScript tests)
• Complete doc strings for new methods.

@Kaiqb as FYI

[coveralls]

Pull Request Test Coverage Report for Build 81234

• 58 of 65 (89.23%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 86.54%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
libraries/botbuilder-core/src/activityHandler.ts	26	27	96.3%
libraries/botbuilder-core/src/activityHandlerBase.ts	31	37	83.78%

Totals
Change from base Build 81220:	0.07%
Covered Lines:	4490
Relevant Lines:	5010

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💛 - Coveralls

[johnataylor]

this is now very close to the C# onMessageActivity ... perhaps in the next iteration we could unify further
(FWIW the introduction of functions here is massively better factoring - much clear to read)

[stevengum]

FYI @daveta @tsuwandy