Implement SSE endpoints following Speakeasy spec

[frankie567]

Speakeasy is supporting SSE endpoints if we define them a certain way in the OpenAPI schema: https://www.speakeasy.com/docs/customize/runtime/server-sent-events

They even support to have schemas for the sent events 🤯

Could be nice to have so developers can easily leverage some of our SSE endpoints (like checkout and benefits stream).