Add configurable diff evaluator

[milesziemer]

Adds a smithy-diff evaluator which can be configured via metadata property diffEvaluators within the model. Unlike the similar EmitEach/EmitNoneSelectorValidators, this diff evaluator is not loaded using SPI, and the ability to create your own custom configurable diff evaluators has not been exposed. Instead, smithy-diff just looks at the new model's metadata and loads any diff evaluators directly. We can expose creating custom configurable diff evaluators later if there is a need, but this diff evaluator should be enough for almost every use case, and we can also extend it easily with more configuration options.

This evaluator works as follows:

1. Get a subset of shapes to match based on the appliesTo property. Currently either added shapes or removed shapes.
2. Optionally filter this subset of shapes further with a selector configured in the filter property, which runs on either the new or old model depending on appliesTo.
3. Run the configured selector on either the new model or old model depending on appliesTo.
4. Match shapes returned by selector to the set of shapes from 2, and emit events based on emitCondition.

This functionality can be extended later by adding more options for emitCondition and appliesTo. For example, there is currently no configuration option for looking at changed shapes, but you could imaging wanting to see which shapes don't match in the old model, but do in the new model, which would be a new appliesTo. Another emitCondition could be IfAnyDontMatch or something.

Also adds a validator to smithy-diff for this metadata property so you'll know when the model is being build if evaluators have been configured incorrectly. This requires having a dependency on smithy-diff in the model package where you're configuring the diff evaluators, but the alternative is stick the validation in smithy-model, creating an implicit dependency on smithy-diff.

See the added test case for an example.

Next Steps:

• Should also add to our docs for this once we know it's concrete.
• Another thing to look at would be moving some of this functionality into smithy-model, since its similar to how the current configurable validators work and we could provide more configuration options for those.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mtdowling]

Can you provide an example of IfAnyMatch that is more realistic?

[mtdowling]

Can we get more removed examples?

[mtdowling]

Is there any use case for IfNoneMatch?

[mtdowling]

Maybe this should emulate validators: https://smithy.io/2.0/spec/model-validation.html#validators. So more like:

{
    id: "AddedInternalOperation"
    name: "EmitEachMatch|EmitIfNoneMatch|EmitIfAnyMatch|..."
    severity: "NOTE"
    configuration: {
        appliesTo: "AddedShapes|RemovedShapes|ChangedShapes"
        selector: "operation [trait|internal]"
    }
}

This keeps the door open for future configurable diff evaluators, and maybe even the ability to configure built-in evaluators. This could also, if we really look into the future, potentially cover metadata diffs and trait diffs too.