caddy: add files required by contemporary slapos

software/caddy/instance-input-schema.json

+{
+  "$schema": "http://json-schema.org/draft-04/schema#",
+  "type": "object",
+  "description": "Parameters to instantiate Caddy",
+  "additionalProperties": false,
+  "properties": {
+  }
+}

software/caddy/instance-output-schema.json

+{
+  "$schema": "https://json-schema.org/draft/2019-09/schema",
+  "description": "Values returned by Caddy instantiation",
+  "additionalProperties": false,
+  "properties": {},
+  "type": "object"
+}

software/caddy/software.cfg.json

+{
+  "name": "Caddy",
+  "description": "Caddy",
+  "serialisation": "xml",
+  "software-type": {
+    "default": {
+      "title": "Default",
+      "description": "Caddy",
+      "request": "instance-input-schema.json",
+      "response": "instance-output-schema.json",
+      "index": 0
+    }
+  }
+}