Add task metadata & update template

[jluethi]

This PR is still WIP. Some open questions:

1. I updated how manifest gets downloaded (old link lead to 404). Does it need to get placed in base folder? It's not in the base folder of package.
2. Is it intentional that the package name is not fractal_plantseg_tasks?
3. Re CI: Would get a bit trickier to build with mamba, I guess we can take inspiration from the ilastik task package. For the moment, I turned it off
4. Do we publish a whl to be able to list tasks easily on the overview page? Or should we not publish whls because one can’t really install it from there?
5. One test fails for me locally, but may be related to issues with which tifffile version gets installed:

with TiffWriter(file, **tifargs) as tif:
>           return tif.save(data, shape, dtype, **kwargs)
**E           TypeError: TiffWriter.save() got an unexpected keyword argument 'compression'**
**../../../mambaforge/envs/plant-seg-0.1.2/lib/python3.11/site-packages/tifffile/tifffile.py**:698: TypeError

=> I was using the env creation script, so that apparently runs into this issue

[lorenzocerrone]

This PR is still WIP. Some open questions:

1. I updated how manifest gets downloaded (old link lead to 404). Does it need to get placed in base folder? It's not in the base folder of package.
2. Is it intentional that the package name is not fractal_plantseg_tasks?
3. Re CI: Would get a bit trickier to build with mamba, I guess we can take inspiration from the ilastik task package. For the moment, I turned it off
4. Do we publish a whl to be able to list tasks easily on the overview page? Or should we not publish whls because one can’t really install it from there?
5. One test fails for me locally, but may be related to issues with which tifffile version gets installed:

1. No, it's not needed. In general, the download process is not a very useful quality-of-life improvement. The downloaded file must be transferred back to the local machine for collection via a file upload.
2. No, and ideally I would like to change it fractal_plantseg_tasks
3. Yes, let's do this in a second time
4. We can publish a wheel, but we need to make sure that a pypi installation will not work without some conda deps
5. These tifffile errors randomly happened in the past. We would need to be to be more strict on pinning the tifffile deps.