CLI: support defining custom commands via C++ & Python API

Add support for add-ons to define commands using the new argument
`-c` or `--command`.

Commands behave as follows:

- Passing in a command enables background mode without the need to pass
  in `--background`.
- All arguments following the command are passed to the command
  (without the need to use the `--` argument).
- Add-ons can define their own commands via
  `bpy.utils.register_cli_command` (see examples in API docs).
- Passing in `--command help` lists all available commands.

Ref !119115

source/blender/python/intern/bpy_cli_command.h

@@ -0,0 +1,20 @@
+/* SPDX-FileCopyrightText: 2023 Blender Authors
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
+
+/** \file
+ * \ingroup pythonintern
+ */
+
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern PyMethodDef BPY_cli_command_register_def;
+extern PyMethodDef BPY_cli_command_unregister_def;
+
+#ifdef __cplusplus
+}
+#endif