support custom GeoTIFF file level metadata

[bossie]

Follow-up ticket of Open-EO/openeo-geotrellis-extensions#317: get file-level metadata in GeoTIFFs.

Should be a matter of defining an API (a format option) and mimicking this:

openeo-geopyspark-driver/openeogeotrellis/geopysparkdatacube.py

Line 1915 in 289b6b1

gtiff_options.addHeadTag("PROCESSING_SOFTWARE",softwareversion)

[bossie]

@JorisCod: would a file_metadata format option to mirror bands_metadata do? For example,

{
  "file_metadata": {
    "product_tile": "29TNE",
    "product_type": "LSF monthly median composite for band B02"
  }
}

[JorisCod]

That would be perfect !