Add tests for rule engine

[sugmanue]

Background

Adds tests for the rules engine. The tests validate that basic functionality works as expected. Added as a new package to make it accessible to Smithy code generators to be able to tests these.

Testing

Added a test to validate that model loads without issues, we already have a validator for the tests, EndpointTestsTraitValidator, that validate that those are correct.

Links

• Links to additional context, if necessary
• Issue #, if applicable (see here for a list of keywords to use for linking issues)

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[github-actions]

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