Add feature complete teams client implementation

[emirkmo]

The current teams webhook message sender is extremely limited. At a minimum we need to support basic env vars like HTTP_PROXY.

1. Create a client class that can hold this data
2. Some class_method so it be configured by env vars or explicitly.
3. Add proper session/retry enabled webhook posting using relevant request pars for sending payloads to a teams webhook.
4. Add unit tests for the client
5. Add an integration test if microsoft supports such a thing (else no point in mocking, doesn't test anything).