ext/datadog: Move datadog exporter into separate repo

[codeboten]

Following open-telemetry/opentelemetry-specification#277 (comment), we need to move third party exporters out of the main repo. This was done for the Azure exporter as well #193.

Ideally this exporter and all other exporters will live in https://github.com/open-telemetry/opentelemetry-python-contrib

[majorgreys]

The reason for this makes sense, thank you for addressing it @codeboten.

I wonder what others take is on how such exporters and the contrib repo in general would have its releases managed.

[codeboten]

Fixed by #1258