chore(rename): move llama_stack.distribution to llama_stack.core (#2975)

We would like to rename the term `template` to `distribution`. To
prepare for that, this is a precursor.

cc @leseb

llama_stack/core/utils/serialize.py

@@ -0,0 +1,18 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import json
+from datetime import datetime
+from enum import Enum
+
+
+class EnumEncoder(json.JSONEncoder):
+    def default(self, obj):
+        if isinstance(obj, Enum):
+            return obj.value
+        elif isinstance(obj, datetime):
+            return obj.isoformat()
+        return super().default(obj)