Add Teams' Channel Restored event

[denscollo]

Fixes #2367

Description

This PR adds the Teams' Channel Restored event in teamsActivityHandler class.

Specific Changes

• Updated teamsActivityHandler class to add the new OnTeamsChannelRestoredEvent method.
• Add unit test for the new method in teamsActivityHandler.tests class.

Testing

The following image shows the new event being handled by a test bot.

The next image shows the new test passing.

[stevengum]

Please fix the return docstrings: task is the not the appropriate value

* @returns A task that represents the work queued to execute.

[denscollo]

@stevengum Thanks for the feedback, we just applied it and fix the master merge conflicts.

[stevengum]

One small comment for the new test

[denscollo]

@stevengum We updated the code. Thanks!

[stevengum]

Thanks!