Add mj_markStack and mj_freeStack as public API functions.

Also add asan instrumentation to detect stack frame leakages (i.e. `mj_markStack` without a corresponding `mj_freeStack` in the same caller function).

PiperOrigin-RevId: 562625645
Change-Id: I4e3ff66ca0b9d08ed0a95cef45393db8e3053e22

introspect/structs.py

@@ -3502,6 +3502,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='size_t'),
                  doc='first available mjtNum address in stack',
              ),
+             StructFieldDecl(
+                 name='pbase',
+                 type=ValueType(name='size_t'),
+                 doc='value of pstack when mj_markStack was last called',
+             ),
              StructFieldDecl(
                  name='parena',
                  type=ValueType(name='size_t'),