Add server codegen flag addValidationExceptionToConstrainedOperations (#3803)

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](https://github.com/smithy-lang/smithy-rs/issues/3802

)

Sample `smithy-build-template.json`
```
"plugins": {
            "rust-server-codegen": {
                "service": "ServiceToGenerateSDKFor",
                "module": "amzn-sample-server-sdk",
                "codegen": {
                    "addValidationExceptionToConstrainedOperations": true,
                }
            }
        }
```

---------

Co-authored-by: Fahad Zubair <fahadzub@amazon.com>