support actual default coefficient of sar_backscatter

[soxofaan]

This hardcodes the default value for sar_backscatter to "gamma0-terrain", following the official spec:

openeo-python-driver/openeo_driver/datastructs.py

Line 7 in ca79053

coefficient: Union[str, None] = "gamma0-terrain"

However, on CDSE deployment, we have "sigma0-ellipsoid" as default (and only option actually).
Picking the default value (when user does not explicitly specify coefficient) should follow this instead

[soxofaan]

refs:

• setting the coefficient parameter in sar_backscatter process does not update process graph openeo-web-editor#350
• https://forum.dataspace.copernicus.eu/t/gamma0-terrain-vs-sigma0-ellipsoid-problem-in-web-editor/3345/2

[soxofaan]

Another thing to consider here: on CDSE deployment the option coefficient=null (non-normalized backscatter) from official spec should also be dropped

[soxofaan]

With above commits in various projects, it already works on CDSE dev

https://openeo.dev.warsaw.openeo.dataspace.copernicus.eu/openeo/1.2/processes/backend/sar_backscatter
now declares the coefficient parameter as:

    {
      "default": "sigma0-ellipsoid",
      "description": "The radiometric correction coefficient.\nOn this backend, only the following option is available:\n\n* `sigma0-ellipsoid`: ground area computed with ellipsoid earth model\n",
      "name": "coefficient",
      "optional": true,
      "schema": {
        "enum": [
          "sigma0-ellipsoid"
        ],
        "type": "string"
      }
    },

(note that null is also not possible option anymore)

And the sar_backscatter implementation will actually default to sigma0-ellipsoid if not explicitly specified in the process graph (done by https://github.com/Open-EO/openeo-geotrellis-kubernetes/blob/874a431286c1e7a00b541e08ab21c9b9f723ed2d/openeo-k8s-custom-processes/custom_processes.py#L194 )

[soxofaan]

Once this feature reaches production, the annoying workaround discussed at https://documentation.dataspace.copernicus.eu/Data/SentinelMissions/Sentinel1.html#openeo-processing-options
(code: https://github.com/eu-cdse/documentation/blob/3095de2e02e505f258635cca71885e25c6aa2d0b/Data/SentinelMissions/Sentinel1.qmd#L62-L79 )
can be removed

[soxofaan]

added integration test for this in os_creodias_openeo_k8s too (0d43b6d970e7ac644660b156e066cade03b2a862)

[soxofaan]

• also pick default coefficient when user doesn't explicitly specify sar_backscatter in process graph