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

doc/python_api/examples/bpy.utils.register_cli_command.py

@@ -0,0 +1,43 @@
+"""
+Custom Commands
+---------------
+
+Registering commands makes it possible to conveniently expose command line
+functionality via commands passed to (``-c`` / ``--command``).
+"""
+
+import sys
+import os
+
+
+def sysinfo_command(argv):
+    import tempfile
+    import sys_info
+
+    if argv and argv[0] == "--help":
+        print("Print system information & exit!")
+        return 0
+
+    with tempfile.TemporaryDirectory() as tempdir:
+        filepath = os.path.join(tempdir, "system_info.txt")
+        sys_info.write_sysinfo(filepath)
+        with open(filepath, "r", encoding="utf-8") as fh:
+            sys.stdout.write(fh.read())
+    return 0
+
+
+cli_commands = []
+
+
+def register():
+    cli_commands.append(bpy.utils.register_cli_command("sysinfo", sysinfo_command))
+
+
+def unregister():
+    for cmd in cli_commands:
+        bpy.utils.unregister_cli_command(cmd)
+    cli_commands.clear()
+
+
+if __name__ == "__main__":
+    register()