Update "local" flow for task collection

[tcompa]

With the next fractal-server release, we are introducing a breaking change in the POST /api/v2/task/collect/pip/ endpoint.

1. In general, the endpoint will expect data instead of json, and it will be a multipart/form-data endpoint.
2. For the case of a "pypi-based" collection (namely the one with e.g. `package="fractal-tasks-core") nothing else changes.
3. For the case of a "local-wheel-file" based collection, there are other changes:
   • The package property of the request body must be unset, instead of pointing to the absolute path of the wheel file
   • The packge_version property of the request body must be unset (this was already a requirement)
   • The wheel file should be uploaded as part of the files object. An example (in Python) would be files = {"file": (wheel_file_name, wheel_file_contents, "application/zip")}

Currently available in https://github.com/fractal-analytics-platform/fractal-server/tree/2045-support-uploading-a-wheel-file-directly-for-task-collection

[tcompa]

A small addition:

When pinned_package_versions is set, it must be sent to the API as a JSON string, and not as a JSON object. This holds for both the pypi-based and the local-wheel-based collections.

[tcompa]

New docs, for the record