Handle tools used for images as plugins

[bschwedler]

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I have some mild concerns about how extensible this model of continually embedding more classes for extra settings will be long-term. We may want to approach this as plugins in the future to process and add any arbitrary number of tools.

https://packaging.python.org/en/latest/guides/creating-and-discovering-plugins/

Originally posted by @ianpittwood in #86 (comment)

PRs:

• feat: generic plugin protocol with dgoss as builtin plugin #406
• feat: add ORAS as a builtin plugin #409