Add an introspect module for structs.

Also missing struct fields to the Python bindings that were discovered as a result of the new `introspect` metadata.

PiperOrigin-RevId: 523947396
Change-Id: I367fd2dc8d5fd7e5712a45b70a5c50da51d152e4

introspect/codegen/generate_functions.py

@@ -28,9 +28,9 @@ from introspect import ast_nodes
 from introspect import type_parsing
 from . import formatter
 
-FLAGS = flags.FLAGS
-flags.DEFINE_string('header_path', None, 'Path to the original mujoco.h')
-flags.DEFINE_string(
+_HEADER_PATH = flags.DEFINE_string(
+    'header_path', None, 'Path to the original mujoco.h')
+_JSON_PATH = flags.DEFINE_string(
     'json_path', None,
     'Path to the JSON file representing the Clang AST for mujoco.h')
 
@@ -113,10 +113,10 @@ def main(argv: Sequence[str]) -> None:
   if len(argv) > 1:
     raise app.UsageError('Too many command-line arguments.')
 
-  with open(FLAGS.json_path, 'r', encoding='utf-8') as f:
+  with open(_JSON_PATH.value, 'r', encoding='utf-8') as f:
     root = json.load(f)
 
-  with open(FLAGS.header_path, 'r') as f:
+  with open(_HEADER_PATH.value, 'r') as f:
     visitor = MjFunctionVisitor(f.read())
 
   traverse(root, visitor)