Add support for the meeting start/end events from the Teams channel.

At a high level:

• Add activityHandler for new events
• Schema for important bits of incoming payload
• Adaptive stack trigger to handle the event
• Add trigger to Teams package

Internal spec: https://microsoft.sharepoint.com/:w:/t/skypespacesteamnew/EfjfIuqE4QlDmIs_HYK8zoIBOHd99vLk8K79BIP8wg144g?e=B0jcWy

Tracking Status

Dotnet SDK Issue

• PR
• Merged

Javascript SDK Issue

• PR
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Python SDK Issue

• PR
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Java SDK Issue

• PR
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Package Issue

• PR
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Docs

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