ValidationException protocol tests for restXml

[david-perez]

This repository contains a test suite for the restJson1 protocol with httpMalformedRequestTests that need to be rejected because the request input does not adhere to the modeled constraints. The test suite is under smithy-aws-protocol-tests/model/restJson1/validation.

This test suite has proved to be very helfpul for the implementation of constraint traits in smithy-rs and for me to not deviate in the error message contents from the already existing implementation in smithy-typescript. I presume that the test suite is also normative for the awsJson 1.0 and awsJson 1.1 protocols, since all these protocols share JSON as their serialization format (but it'd be good to explicitly call this fact out in the spec or add protocol tests if there are deviations).

However, there is no reference for the restXml protocol. This issue is a feature request for such test suite to be added.