[CT-3555] Add detect artifact changes workflow

[MichelleArk]

Housekeeping

• I am a maintainer of dbt-core

Short description

We should detect whether a PR makes changes to an existing artifact schema, and fail CI if this is the case. https://github.com/marketplace/actions/prevent-file-change might do the trick!

Or, if we don't want to use an (overly-complex) public action, could do something manually:

• check which files changed, and fail if any of those are modified
• allow new files in dbt/artifacts

Acceptance criteria

• CI fails if any file is modified in dbt/artifacts
• failure message provides details
• CI failure should either: not block merge, or is made to pass by setting a label (e.g. artifact_minor_upgrade)

Future work could be only failing the check if breaking changes are introduced. Until then, we can manually inspect the changes and merge on red if the change is deemed safe.

Additionally, tests in dbt/artifacts should provide signal as to whether a breaking change is made to an existing artifact because existing upgrade/downgrade logic would fail.

Impact to Other Teams

N/A - CI work in dbt-core