wcst_import - supports auto option to fetch metadata for general_recipe with slicer gdal

[Bang Pham Huu]

Currently, only general_recipe with slicer: netCDF supports the feature to fetch metadata implicitly, e.g:

        "metadata": {
          "type": "xml",
          "global": "auto"
	},

In case of importing Tiff with tags file by general_recipe and slicer gdal, it would be good to collect them automatically as well.