Refactor Teams SSO middleware

[sw-joelmut]

Related issue #6861
#minor

Description

This PR refactors the Teams SSO middleware to fix an issue with the usage of BlobsStorage.
When using BlobsStorage in the middleware, by providing an ETag to an item that will be saved for the first time, it throws an HTTP 412 error, then being captured as an ETag Exception and the conversation gets stuck.
The main reason of why this middleware was saving stuff in the storage was to avoid duplicated requests (when having more than one MS Teams client sign-in), process the first and discard the rest.

We ended up refactoring the middleware to use the storage ETag instead of custom one, and track the deduplication process per user per conversation instead of the token exchange request id that comes in the Activity.Value.

Specific Changes

• Changed middleware documentation to understand better what this middleware does.
• Changed the Storage key format to <channelId>/<conversation id>/<user id> to avoid colliding with the same user in other conversations.
• Added ETagException to handle HTTP 412 errors better, it also includes the "ETag conflict" text at the start to maintain compatibility.
• Adjusted one unit test to use concurrency.

Testing

The following image shows an example of the new implementation.