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

scripts/modules/bpy/utils/__init__.py

@@ -21,6 +21,8 @@ __all__ = (
     "refresh_script_paths",
     "app_template_paths",
     "register_class",
+    "register_cli_command",
+    "unregister_cli_command",
     "register_manual_map",
     "unregister_manual_map",
     "register_classes_factory",
@@ -49,9 +51,11 @@ from _bpy import (
     flip_name,
     unescape_identifier,
     register_class,
+    register_cli_command,
     resource_path,
     script_paths as _bpy_script_paths,
     unregister_class,
+    unregister_cli_command,
     user_resource as _user_resource,
     system_resource,
 )