feat: add error-on-diff flag for summary command

[mattiZed]

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Hey!

This is an implementation of what I opened as an issue in #198. Adding the --error-on-diff flag to the summary command will make the tool terminate with an error on any (be it non-breaking or breaking) differences. This basically has the same effect as the --fail-on-diff flag in oasdiff tool.

Let me know your thoughts.