feat: add --playground flag to cog serve (#3154)
* feat: add --playground flag to cog serve
Start the playground alongside the model on a dedicated port, print both
URLs, and shut it down cleanly on Ctrl+C. The playground port never
collides with the model port.
* fix: point playground at the model's real bind host
Use the actual serve host instead of hardcoding localhost, so the
playground proxy matches the address the model is really bound to.
* fix: support playground webhooks from model containers
