[Python] Support event streaming trait

[crisidev]

Support Smithy event streaming trait.

[XiangyangShi]

Plus one on this.
Yes please!

Wanna share more a little bit on this, and why this item is important:
Our team is building a machine learning service which support streaming inference on an RNN model.
Since its a machine learning model, we have to use python server generation feature of smithy.
Because of streaming inference (instead of batch inference) we need to have EventStreaming.

[crisidev]

Now that union support have been merged (#2427) we are confident we can deliver event streaming support. I don't have a timeline, but should happen soon.