Support for rpi-imager: Selection of DietPi

[StephanStS]

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Needed for rpi-imager integration:

• DietPi logo
• .json file (testwise extended, needs to be reduced later). File can be renamed to dietpi_list.json

To request DietPi to be integrated into rpi-imager OS list, these files need to be supplied.

Local usage (with extended .json file): rpi-imager --repo custom_os_list.json