Provide S2Downloader as a native conda package

So far, the installation of S2Downloader relies on an environment.yml file that defines the package requirements. The actual S2Downloader code is then installed via pip on top of the resulting conda environment. This works but is not very handy if users want to integrate S2Downloader just as another package as part of their conda environment.

To improve that, S2Downloader should be provided as a native conda package downloadable and installable from the Anaconda Cloud (via conda install s2downloader). This is what I already did a couple of times, e.g., for arosics which is installable from the conda-forge conda channel, see https://anaconda.org/conda-forge/arosics and https://github.com/conda-forge/arosics-feedstock. Basically, you need to create a feedstock repository via a pull request to the staged-recipes repository and then maintain the feedstock in case of updates. The latter is quite automated, so there is no not much work to be done there. Further details are provided here.

I would of course offer my help and could also initiate the feedstock creation but its maintenance should be mainly on your hand afterwards.