Add Exiftool in list of software that can help handle metadata

[karlheger]

How could the content be improved?

For handling image metadata the Exiftool is in my opinion the most powerful tool as it understands most metadata formats and is solely dedicated to handling image metadata. So I think it might be worth mentioning it in the Accessing Metadata section as well.

Which part of the content does your suggestion apply to?

https://github.com/datacarpentry/image-processing/blob/main/episodes/02-image-basics.md