[Feature] Support local files (jar and/or signature)

[thomasleplus]

Feature description

It could be useful to be able to validate locale artifacts instead of downloading them from a repo.

For example we could introduce two new CLI options:
- `--artifact-url` to specify the full URL where to download the artifact.
- `--signature-url` to specify the full URL where to download the signature.

Both options should support the `file:` URL scheme.

Both options would be incompatible with the `--repo-base-url` option that is used today to derive the URLs.

Anything else?