Add DB providers command

[wagoodman]

It should be trivial to see what data providers (from vunnel) populated the DB, along with timestamp information:

grype db providers

alpine       2024-09-16T01:34:04.087474Z      ...
amazon       2024-09-16T01:34:04.087474Z      ...
chainguard   2024-10-16T01:34:04.087474Z      ...
debian       2024-09-16T01:34:04.087474Z      ...
github       2024-09-28T01:34:04.087474Z      ...
mariner      2024-09-16T01:34:04.087474Z
nvd          2023-11-16T01:34:04.087474Z
oracle       2024-09-16T01:34:04.087474Z
rhel         2024-09-16T01:34:04.087474Z
sles         2024-09-16T01:34:04.087474Z
ubuntu       2024-09-16T01:34:04.087474Z
wolfi        2024-09-16T01:34:04.087474Z

This should be available as:

• table output
• json output

Example JSON output:

{
  "providers": [
    {
      "name": "nvd",
      "dateCaptured": "2024-09-16T01:34:04.087474Z",
      ...
    },
    ...
  ]
}

Ideally we'd capture vunnel version information either in bulk or with each provider. For instance, see the prototype information capture here: https://github.com/anchore/grype/blob/db-v6-blob-store/grype/db/v6/models.go#L71-L95