[Read] Add type stac to actinia importer

[mmacata]

Piece of process chain should look similar like this:

        {
            "id": "importer_1",
            "module": "importer",
            "inputs": [
                {
                    "import_descr": {
                        "source": "stac.example-instance-id.rastercube.example-collection-id",
                        "type": "stac",
                        "semantic_label": "red",
                        "extent": {
                            "spatial": {
                                "bbox":[[-180,-90,180,90]]
                            },
                             temporal":{"interval":[[
                                   "2021-09-09", "2021-09-12"
                             ]]}
                         },
                         "filter": {...}
                    },
                    "param": "map",
                    "value": "example-red"
                }
            ]
        },

• Why "semantic_label" ? -> [Bug] Band references need a better name and definition OSGeo/grass#1868
• Why "red" ? See https://github.com/stac-extensions/eo/#common-band-names - Although the names are given by the STAC catalog itself and don't always follow the common band names guideline
• Why extent? -> https://openeo.org/documentation/1.0/developers/api/reference.html#operation/describe-collection
• Why filter? -> More filter options need to be implemented step by step without breaking the API all the time, so they should be collected in a single object or array.

Currently there is the parameter sentinel_band available in the importer. Although it would make sense to also use semantic_label for it, we don't touch it for now.

The goal of this ticket is not a full implementation to make this PC work but only a preparation to accept the PC, acknowledge the type stac and prepare for semantic_label, extent and filter. When using type stac in a process chain, actinia shouldn't start processing but return a message like "STAC import is coming soon" or similar.

Hint: the importer part of a process chain can also be used directly with a module, e.g.

    {
      "module": "g.region",
      "id": "g_region_1",
      "inputs": [
        {
          "import_descr": {
            "source": "https://storage.googleapis.com/graas-geodata/elev_ned_30m.tif",
            "type": "raster"
          },
          "param": "raster",
          "value": "elev_ned_30m_new"
        }
      ],
      "flags": "p"
    },

I suspect that this will work out of the box as well (as same code is executed) but it should be tested as well.

Also update API docs accordingly and prepare tests.