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/llama.py

@@ -9,6 +9,7 @@ import argparse
 from .download import Download
 from .model import ModelParser
 from .stack import StackParser
+from .verify_download import VerifyDownload
 
 
 class LlamaCLIParser:
@@ -27,9 +28,10 @@ class LlamaCLIParser:
         subparsers = self.parser.add_subparsers(title="subcommands")
 
         # Add sub-commands
-        Download.create(subparsers)
         ModelParser.create(subparsers)
         StackParser.create(subparsers)
+        Download.create(subparsers)
+        VerifyDownload.create(subparsers)
 
     def parse_args(self) -> argparse.Namespace:
         return self.parser.parse_args()