gh-158572: Add an external flag to interpreter frames (#158573)

Adds an `external` flag to a bit-packed byte on the
_PyInterpreterFrame struct in order to provide external frame
evaluators a way to distinguish VM-pushed frames vs externally-
pushed frames.

Include/internal/pycore_interpframe_structs.h

@@ -42,12 +42,11 @@ struct _PyInterpreterFrame {
 #endif
     uint16_t return_offset;  /* Only relevant during a function call */
     char owner;
-#ifdef Py_DEBUG
     uint8_t visited:1;
+    uint8_t external:1; /* Set when the frame was pushed by something other than the VM */
+#ifdef Py_DEBUG
     uint8_t stackpointer_valid:1;
-    uint8_t lltrace:6;
-#else
-    uint8_t visited;
+    uint8_t lltrace:5;
 #endif
     /* Locals and stack */
     _PyStackRef localsplus[1];