Add a verify-download command to llama CLI (#457)

# What does this PR do?

It is important to verify large checkpoints downloaded via `llama model
download` because subtle corruptions can easily happen with large file
system writes. This PR adds a `verify-download` subcommand. Note that
verification itself is a very time consuming process (and will take
several **minutes** for the 405B model), hence this is a separate
subcommand (and not part of the download which can already be
time-consuming) and there are spinners and a bit of a "show" around it
in the implementation.

## Test Plan

<img width="1012" alt="image"
src="https://github.com/user-attachments/assets/f82b0d42-2a15-4917-b85e-6d3cd7d31e55">

llama_stack/cli/model/verify_download.py

@@ -0,0 +1,24 @@
+# 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 argparse
+
+from llama_stack.cli.subcommand import Subcommand
+
+
+class ModelVerifyDownload(Subcommand):
+    def __init__(self, subparsers: argparse._SubParsersAction):
+        super().__init__()
+        self.parser = subparsers.add_parser(
+            "verify-download",
+            prog="llama model verify-download",
+            description="Verify the downloaded checkpoints' checksums",
+            formatter_class=argparse.RawTextHelpFormatter,
+        )
+
+        from llama_stack.cli.verify_download import setup_verify_download_parser
+
+        setup_verify_download_parser(self.parser)