MTG - add day/night flag

[LucaBarco]

Right now, the day/night flg is set to null for MTG granules. This has to be fixed because it is needed for composite selection.

Example of _granule.json

{ "granule_id": "W_XX-EUMETSAT-Darmstadt,IMG+SAT,MTI1+FCI-1C-RRAD-FDHSI-FD--x-x---x_C_EUMT_20250615211315_IDPFI_OPE_20250615211007_20250615211934_N__O_0128_0000", "source": "EO:EUM:DAT:0662", "assets": { "product": { "href": "https://api.eumetsat.int/data/download/1.0.0/collections/EO%3AEUM%3ADAT%3A0662/products/W_XX-EUMETSAT-Darmstadt%2CIMG%2BSAT%2CMTI1%2BFCI-1C-RRAD-FDHSI-FD--x-x---x_C_EUMT_20250615211315_IDPFI_OPE_20250615211007_20250615211934_N__O_0128_0000?access_token=69f117a3-fecc-3f41-82fe-b125b2b0f404" } }, "info": { "instrument": "FCI", "level": "", "product_type": "MTIFCI1CRRADFDHSI", "acquisition_time": "2025-06-15T21:19:34" }, "local_path": "samples_MTG/W_XX-EUMETSAT-Darmstadt,IMG+SAT,MTI1+FCI-1C-RRAD-FDHSI-FD--x-x---x_C_EUMT_20250615211315_IDPFI_OPE_20250615211007_20250615211934_N__O_0128_0000.MTG", "day_night_flag": null }