Add optional `path` argument to `Flowmap.to_randonneur` and `Flowmap.to_glad`

[fjuniorr]

If the path is provided, the mapping files are serialized directly to disk, instead of being returned. This will both simplify:

https://github.com/fjuniorr/flowmapper/blob/03e2418e76cb520c82ff5e71ad9a188e8b7aae63/flowmapper/cli.py#L84-L92

and make this functionality available outside of the CLI (for example for easily generating this files from flowmapper-ci/status.qmd)