feat: add option to opt-in free-threaded builds

bin/generate_schema.py

@@ -104,6 +104,10 @@ properties:
     description: Set environment variables on the host to pass-through to the container
       during the build.
     type: string_array
+  free-threaded-support:
+    type: boolean
+    default: false
+    description: The project supports free-threaded builds of Python (PEP703)
   manylinux-aarch64-image:
     type: string
     description: Specify alternative manylinux / musllinux container images
@@ -248,6 +252,7 @@ non_global_options = {k: {"$ref": f"#/properties/{k}"} for k in schema["properti
 del non_global_options["build"]
 del non_global_options["skip"]
 del non_global_options["test-skip"]
+del non_global_options["free-threaded-support"]
 
 overrides["items"]["properties"]["select"]["oneOf"] = string_array
 overrides["items"]["properties"] |= non_global_options.copy()