[rust-server] Restore newtype and XML support

Created by: bjgill

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

As part of the migration from swagger-codegen to openapi-generator, the rust-server generator lost support for newtypes (e.g. pub struct OuterString(String)). Partly as a consequence, XML support was also broken.

This PR undoes the change made by the default generator to unwrap references. This restores the old behaviour of rust-server.

I've added in a sample to verify the XML behaviour. We've also successfully used this branch to generate a Rust client for one of our services currently migrating from swagger-codegen to openapi-generator.

This will be my final PR to openapi-generator on behalf of @metaswitch. I am leaving the company at the end of the year. @rjw2 will be taking over my role in handling upstream contributions

Fixes #1590