REUSE metadata traversal

There should be a command in format:
```console
$ dotnet authors --source reuse --target json --output authors.json
```
(other potential `--source` in the future will be `git`, and other potential `--target` will be `reuse`)

Which should collect the copyright holders from the REUSE files according to the config (if passed) and produce the `authors.json` file.

Resulting data format example:
```json
{ "authors": [{
"name": "Friedrich von Never",
"contact": [{
"url": "mailto:friedrich@fornever.me"
}, {
"info": "any optional text info"
}]
]}}
```

`contact` may consist of different entries in case there are different copyright entries in different files.

URLs should be valid (thus dotnet-authors should recognize at least `mailto:` and `https://` correctly when needed).

In the future, config will help to deduplicate authors.

There's nothing wrong in exposing the author info this way since they are stored in the REUSEable sources anyway.

Start at the CLI entry point for the `dotnet authors` command and trace how command options and output targets are handled. Implement the `--source reuse --target json --output authors.json` flow using REUSE copyright entries, with the shown authors/contact JSON shape and support for `mailto:` and `https://` URLs. Done means the command produces the expected file from configured REUSE metadata.

Written by the indexing model from the issue text.