Add support for Shared channels in MS Teams integration

### Problem Statement

As a developer, I would like to send alerts to MS Teams, to a specific group of people. Teams supports [different types of channels](https://support.microsoft.com/en-us/office/teams-can-have-standard-private-or-shared-channels-de3e20b0-7494-439c-b7e5-75899ebe6a0e):
* Standard
* Private
* Shared

Currently Sentry integration for Teams only supports Standard channels. These channels are available to all members of a team. But for Sentry alerts, we would like to limit access to only certain members of a team. Looks like Private channels cannot be used for this purpose, because as mentioned on the link above:
> Wikis, apps and bots, and scheduled channel meetings aren't supported at this time.

Probably Shared channels could be used, but it would require Sentry to support them.

### Solution Brainstorm

Add support for Shared channels in Teams integration, in addition to Standard channels.