Add jsonschema-tools diff subcommand
Created by: ottomata
This finds materialized schema versions that have been modified since the previous commit, and then prints a diff between them.
This will make reviewing schema changes easier. We can schedule jsonschema-tools diff to be run as a CI job, and reviewers can look at the output to compare e.g. version 1.0.0 to version 1.1.0, and so on.
In WMF CI, we should use dyff as the --diff-command to show semantic yaml diffs, instead of textual diffs.
Bug: T321850