Support extraBody request in payloads

[triozer]

Context

This makes the client more flexible when callers need to send auxiliary JSON-safe data alongside the normal FAL input payload.

This is especially useful when using fal behind a proxy. The proxy already receives and forwards the JSON request body, so a dedicated extraBody field gives applications a clean place for proxy-specific metadata or control values without overloading the model’s top-level input schema. That makes it easier for proxy code to inspect, transform, strip, or act on those fields before forwarding the request upstream.

What Changed

• add extraBody to client request options for run, queue submit/subscribe, and streaming
• serialize non-GET payloads as { ...input, extraBody } while keeping extraBody JSON-safe and untransformed
• add focused tests for request body shaping, streaming parity, and invalid GET / non-object input cases