MTG - Include option to download and process HRFI data

[edornd]

Moderate and high resolution bands belong to different product IDs in EUMETSAT:

• https://user.eumetsat.int/catalogue/EO:EUM:DAT:0665 for HRFI
• https://user.eumetsat.int/catalogue/EO:EUM:DAT:0662 for FDHSI

This requires two separate downloads, on top of that some composites only work with FDHSI or HRFI (see github.com/pytroll/satpy/issues/3076)

Ideas to handle this:

1. add an option in the source init to download either low or high resolution, abstracting the double product in the source
2. double the source, make one specific for low res, and one specific for high res.

[AndreaBrg]

Shouldn't the user provide the correct composite(s) based on the requested product? This is how MODIS/VIIRS work, and they too, especially VIIRS, need specific composites based on the product (e.g., IMG vs MOD).