[Rust] Ignore additional properties

Created by: bjgill

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• 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

This is a workaround for https://github.com/OpenAPITools/openapi-generator/issues/318. In swagger-codegen, we did not support additionalProperties and so ignored them.

In openapi-generator, we now get back a type (map), where we got back a null previously. postProcessModels doesn't have enough context to work out what sort of map is required (we don't know the types of the additionalProperties).

Thus, this workaround detects this specific failure case and fiddles with the dataType to get additionalProperties ignored again. This will allow us to generate from specs containing additionalProperties and have the resultant crates compile.

I've added in a warning to users to let them know what's happening.

I'm building on top of https://github.com/OpenAPITools/openapi-generator/pull/658 to allow us to add a test to ensure that we don't regress this in future.