[Feature] Add Microsoft Teams channel plugin

[arefbehboudi]

Add a Microsoft Teams channel plugin, following the existing channel plugin pattern
(see plugins/telegram and plugins/discord).

Scope:

• New plugins/teams module with TeamsChannel, TeamsChannelAutoConfiguration,
  and TeamsOnboardingProvider.
• Receive messages from Teams and reply with agent responses, like the other channels.
• Onboarding should collect the required Bot Framework credentials
  (App ID, App Secret, Tenant ID).

Note: unlike Telegram/Discord, Teams bots receive messages via a public HTTPS
webhook (Azure Bot Service), so the setup docs should mention exposing an
endpoint (e.g. via a tunnel) for self-hosted installs.