add option to use different s3 settings for inputs and output

[ungarj]

e.g. to enable reading and writing using different access keys or even s3 backends

...
input:
  s2:
    format: S2AWS_COG
    start_time: 2018-04-02
    end_time: 2018-04-03
    read_options:  # for input drivers using rasterio and/or fiona, this mapping should be passed on in rasterio.Env or fiona.Env
        AWS_ACCESS_KEY_ID: ENV.SRC_AWS_ACCESS_KEY_ID  # read SRC_AWS_ACCESS_KEY_ID from environment variable
        AWS_DEFAULT_REGION="eu-central-1"  # use this string
        AWS_SECRET_ACCESS_KEY=ENV.SRC_AWS_SECRET_ACCESS_KEY  # read SRC_AWS_SECRET_ACCESS_KEY from environment variable
...
output:
  format: GTiff
  bands: 3
  path: s3://some-bucket/
  dtype: uint8
  write_options:  # for output drivers using rasterio and/or fiona, this mapping should be passed on in rasterio.Env or fiona.Env
      AWS_ACCESS_KEY_ID: ENV.DST_AWS_ACCESS_KEY_ID  # read DST_AWS_ACCESS_KEY_ID from environment variable
      AWS_S3_ENDPOINT_URL="https://obs.some-provider.com/"  # use this string
      AWS_SECRET_ACCESS_KEY=ENV.DST_SECRET_ACCESS_KEY  # read DST_SECRET_ACCESS_KEY from environment variable
...

This should work for drivers using rasterio and/or fiona but we have to make sure, these mappings are passed on to every rasterio/fiona call. Likewise drivers using different packages like xarray should be provided also with this mapping if possible. Hence the generic names read_options and write_options.

Note that it should be possible by either providing these settings in the configuration as raw string or as a reference to an environment variable.