Add exec output-schema parameter (#4079)

Adds structured output to `exec` via the `--structured-output`
parameter.

codex-rs/exec/tests/suite/mod.rs

@@ -1,5 +1,6 @@
 // Aggregates all former standalone integration tests as modules.
 mod apply_patch;
 mod common;
+mod output_schema;
 mod resume;
 mod sandbox;