Add option for custom FITS headers

[Thomas Birbacher]

It would be great to be able to set user-customisable FITS header information in NINA that is then saved with each exposure. E.g. let the user set keywords like “OBSERVER=Name of person” or instrument configurations that are not picked up by the drivers/ASCOM (like manual steps such as adjusting a filter or mask that is not electronically controlled). This would be useful metadata for data processing and organisation.