Transform to add a shape to an operation's errors

[david-perez]

It is somewhat common for smithy-rs server users to want to generate an SDK from an existing Smithy model and get:

[SEVERE] Operation com.amazon.service#Operation takes in input that is
constrained
(https://awslabs.github.io/smithy/2.0/spec/constraint-traits.html), and as
such can fail with a validation exception. You must model this behavior in
the operation shape in your model file.
```smithy
use smithy.framework#ValidationException

operation Operation {
    ...
    errors: [..., ValidationException] // <-- Add this.
}

To fix this and still have the framework honor validation, they'll have to create a new model, import the existing one, and re-define all the constrained operations' errors.

This can be a bit boilerplatey, and inconvenient when you don't own the model, so perhaps we should vend a transform that adds a given shape to an operation's list of errors.

[david-perez]

The code for this transform could start off https://github.com/smithy-lang/smithy-rs/blob/main/codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/transformers/AttachValidationExceptionToConstrainedOperationInputsInAllowList.kt