Use the cloud snapshots manifest in Mithril Aggregator

[jpraynaud]

Implementation of list_snapshots and use it in http handler snapshots with @Alenar.
Relates to #120

Next steps:

• Implement get_snapshot_details and use it in http handler snapshot_digest
• Enhance error output by using entities::Error
• Inject dependency of a SnapshotStorer and use of a configuration file

[github-actions]

Unit Test Results

1 files  1 suites   1s ⏱️
5 tests 5 ✔️ 0 💤 0 ❌

Results for commit 6ecf689.

♻️ This comment has been updated with latest results.

[ghost]

The bucket's name is likely to change and not reflect the network name but that's a good start!