Add script that diffs event versions

[magnusbaeck]

Description

We should add a script that diffs the currently latest version of each type against the latest version at some other point in time, e.g. the current state of the upstream branch. This could either be done locally but would also be run automatically when a PR is uploaded, and the result could be posted as a PR comment.

For example, for a PR that adds a new ArtC version you'd be interested in the output of the following commands:

diff -u definitions/EiffelArtifactCreatedEvent/3.1.0.yml definitions/EiffelArtifactCreatedEvent/3.2.0.yml
diff -u schemas/EiffelArtifactCreatedEvent/3.1.0.json schemas/EiffelArtifactCreatedEvent/3.2.0.json

Motivation

Reviewing commits that add a new event version is tedious today since it's a brand new file that needs to be compared to some other file in the same directory. This needs to be done by fetching the PR branch and diffing the files locally. This is extremely tedious if the PR modifies the meta members.

Exemplification

N/A

Benefits

Quality of life improvement for reviewers and those pushing commits since it's easier for them to validate their work.

Possible Drawbacks

Could be chatty if we post a comment every time a commit is pushed to a PR branch (if the PR branch touches definitions//.yml anyway).