Autogenerate project schema

[mr-cal]

What needs to get done

Add a CI job that generates the schema. When a PR changes the schema, a CI job should fail.

Rockcraft provides a good reference. Charmcraft will be adding the same functionality soon, Snapcraft can leverage Charmcraft's work too.

This requires #4885 to be completed first.

Why it needs to get done

So the schemastore has an up-to-date copy of Snapcraft's schema.

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-4131.

This message was autogenerated