Improve proxy support for the federation_client.py dev script

[dkasak]

This PR does two things:

1. It makes the federation-client script respect HTTP_PROXY, HTTPS_PROXY
   and similar environment variables.
2. It ensures most requests (in particular those managed by the
   MatrixConnectionAdapter) actually go through the proxy.

Signed-off-by: Denis Kasak [email protected]

Pull Request Checklist

• Pull request is based on the develop branch
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[anoadragon453]

Thanks!