provide external source control URL for schema publish

We already support linking github metadata with a schema publish via the `--github` flag.

However, not everyone is using GitHub. For these users we can provide a different flag for associating a link with a subgraph publish.

```bash
hive schema:publish \
--commit c7cd95c28de78a92de2250ed34c2aa8fb987ffc3 \
--external-url https://codeberg.org/foo/bars/commit/c7cd95c28de78a92de2250ed34c2aa8fb987ffc3 \
# ... other parameters
```

We can then leverage that link in the UI to give users a quick way to jump to their source control provider.

____

Consideration:
- The URL path bath must be allow-listed in the target/project/orgs settings by an admin; to avoid sus links being displayed to users