Add API call to download one file

Currently, there's no simple way to fetch just one file from a project. There is download_file() API call, but it needs the project to be already downloaded (it is useful just to get older versions of a file).

One could either query MM server API directly, e.g.:

https://app.merginmaps.com/v1/project/raw/<workspace>/<project>?file=myfile.gpkg

(with appropriate auth token)

Alternatively, using python with a workaround like this (handles auth stuff):

from mergin import MerginClient
from mergin.utils import save_to_file

mergin_client = MerginClient(...)
res = mergin_client.get('/v1/project/raw/<workspace>/<project>', data={'file': 'myfile.gpkg'})
save_to_file(res, 'myfile.gpkg')

If we add support for this, maybe we could just update the existing download_file() implementation so that it will accept as the first argument either <workspace>/<project> or it would accept an existing the local directory with a project downloaded already.

No project checkout, download raw file