feat(clients): use schema-serde in AWS REST XML clients

[kuhe]

Issue

smithy-lang/smithy-typescript#1600

Description

cuts AWS REST XML clients over to schema serde

Testing

• we have a large existing S3 test suite

Additional context

release group:
#7483 JSON RPC
#7485 REST JSON
#7489 Query & EC2
#7499 REST XML

Checklist

• If the PR is a feature, add integration tests (*.integ.spec.ts).
• If you wrote E2E tests, are they resilient to concurrent I/O?
• If adding new public functions, did you add the @public tag and enable doc generation on the package?