Author metadata verificatiton

There should be a command like
```console
$ dotnet authors --source reuse --target json --output authors.json --verify
```

This command should not update the output file, but should verify its contents. If there's a contradiction — print a patch to the `.json` file and terminate with a non-zero exit code (meant for use in CI).

Depends on:
- [ ] #82

Start by reading the existing authors command and the work tracked in dependency #82. Add the --verify behavior for JSON output: leave the output file unchanged, print a patch when contents contradict the source, and exit with a non-zero status for CI use.

Written by the indexing model from the issue text.