[SECRES-3945] Separate manifest file and samples sync workflows

[ikretz]

This PR separates the samples syncing and manifest file syncing into two separate workflows.

A new script, scripts/sync-manifest/ has been added, along with a corresponding new workflow, .github/workflows/sync-manifest.yaml. As an initial trial period, this workflow is set to run every 2 hours on weekdays from 9h-17h UTC, with a manual PR approval required. The workflow runs the new sync_manifest script to sync the manifest files directly with the backend.

Other changes include:

• Remove the scripts/generate_manifest/ script that was previously responsible for generating the manifest files from the dataset contents
• Update the sync-malicious-packages workflow to no longer perform any manifest-related operations
• Perform a one-off, all-time sync of the manifests with the backend (picked up several hundred new items)
• Remove a sample npm/compromised-libs/xrlp/4.3.0 that should not have been included in the dataset