Add server codegen flag addValidationExceptionToConstrainedOperations

[drganjoo]

The Server SDK requires the model to include aws.smithy.framework#ValidationException in each operation that can access a constrained member shape. This becomes problematic when generating the server SDK for a model not owned by the team creating the SDK, as they cannot easily modify the model.

This PR introduces a codegen flag, addValidationExceptionToConstrainedOperations. When set in smithy-build-template.json, this flag will automatically add ValidationException to operations that require it but do not already list it among their errors.

Closes Issue: 3802

Sample smithy-build-template.json

"plugins": {
            "rust-server-codegen": {
                "service": "ServiceToGenerateSDKFor",
                "module": "amzn-sample-server-sdk",
                "codegen": {
                    "addValidationExceptionToConstrainedOperations": true,
                }
            }
        }

[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.

[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• Client Test (ignoring whitespace)
• Server Test (ignoring whitespace)
• Server Test Python (ignoring whitespace)
• Server Test Typescript (ignoring whitespace)

A new doc preview is ready to view.

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• Client Test (ignoring whitespace)
• Server Test (ignoring whitespace)
• Server Test Python (ignoring whitespace)
• Server Test Typescript (ignoring whitespace)

A new doc preview is ready to view.

[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.