Implement gdb.execute_mi

This adds a new Python function, gdb.execute_mi, that can be used to
invoke an MI command but get the output as a Python object, rather
than a string.  This is done by implementing a new ui_out subclass
that builds a Python object.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=11688
Reviewed-By: Eli Zaretskii <eliz@gnu.org>

gdb/python/python-internal.h

@@ -483,6 +483,9 @@ struct symtab_and_line *sal_object_to_symtab_and_line (PyObject *obj);
 frame_info_ptr frame_object_to_frame_info (PyObject *frame_obj);
 struct gdbarch *arch_object_to_gdbarch (PyObject *obj);
 
+extern PyObject *gdbpy_execute_mi_command (PyObject *self, PyObject *args,
+					   PyObject *kw);
+
 /* Convert Python object OBJ to a program_space pointer.  OBJ must be a
    gdb.Progspace reference.  Return nullptr if the gdb.Progspace is not
    valid (see gdb.Progspace.is_valid), otherwise return the program_space