Downgrade additionalProperties violations to warnings

[lcarva]

When validating rule data, we often set additionalProperties to false, see example. The value of doing this is that it allows us to catch typos on non-required properties.

However, when we want to add a new property, we must carefully synchronize changes. After all, older versions of ec-policies will emit a violation for such property.

Let's introduce a mechanism to bring the best of both sides. Instead of emitting a violation when an unknown property is used, let's emit a warning.

Because this pattern will have to me used in multiple places within this repo, it's a good idea to create some sort of helper rule for this.

To avoid duplicating policy rules (one for schema violations and one for schema warnings), let's leverage enterprise-contract/ec-cli#2025 for this work.

Acceptance Criteria

• When unknown properties are used for rule data items, a warning is emitted instead of a violation.