Make aux-data accessible

[forman]

We need way(s) to provide required aux-data for the xcube data store smos:

1. SMOS discrete global grid data. We currently rely on a version installed
   by SNAP plugin SMOS-Box in ~/.snap/auxdata/smos-dgg/grid-tiles.
   We currently set its path via an
   environment variable XCUBE_SMOS_DGG_PATH.
2. SMOS NetCDF Kerchunk index. We currently set its path via an
   environment variable XCUBE_SMOS_INDEX_PATH.

Here are a number of options. They could also be provided in combination.

• Put aux-data in publicly available S3 bucket (slower).
• Provide installable aux-data package in conda/pip (faster).
• Bundle aux-data with container image.
• Put aux-data in publicly available FTP for download and configure store
  via env var XCUBE_SMOS_INDEX_PATH.