[#712] Add TeamsAttachmentDownloader to Core

[ceciliaavila]

Fixes #712

Description

This PR moves the TeamsAttachmentDownloader class from agents-hosting-extensions-teams project to agents-hosting/app.

Detailed Changes

• Moved the TeamsAttachmentDownloader class to agents-hosting/app.
• The TeamsAttachmentDownloader class within the Teams extensions has been marked as obsolete. Its has been extended from the new class, and the previous implementation has been removed.
• Improved the downloadFile function adding validations, handling errors, an making the logic closer to the one in .NET.
• Added a channel validation in the downloadFiles function.
• Added M365Copilot to Channels list.
• Updated the samples/teams/teamsAttachments sample to use the new class.

Testing

The following images show the teamsAttachments sample working after the changes.

[benbrown]

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[ceciliaavila]

left a few comments

Hi @benbrown, thanks for the feedback! I answered your comments and applied a small change.