Stream response schemas

Add schema for the data shapes in the streaming endpoints.

- API docs: https://docs.joinmastodon.org/methods/streaming/
- Example SSE implementation in OpenAPI: https://www.speakeasy.com/docs/sdks/customize/runtime/server-sent-events

The content type should be updated to `text/event-stream`. Implement schemas similar to "Handling Multiple Event Types" from the example link.