feat(extensions): implement initial extension feature and first extension

Signed-off-by: Dominik Willner <th33xitus@gmail.com>

kiauh/extensions/gcode_shell_cmd/metadata.json

@@ -0,0 +1,9 @@
+{
+  "metadata": {
+    "index": 1,
+    "module": "gcode_shell_cmd_extension",
+    "maintained_by": "dw-0",
+    "display_name": "G-Code Shell Command",
+    "description": "Allows to run a shell command from gcode."
+  }
+}