[Rust Server] Support types with additional properties

Created by: richardwhiuk

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, ./bin/openapi3/{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\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.

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  • @frol, @farcaller, @bjgill

Description of the PR

This adds support for types which only contain additional properties, which is a common use case. It doesn't handle the thornier case of additional properties within a type with defined properties, which is allowed by the OpenAPI spec, but is more dubious API design in practice (as the API is mixing schema-ed and un-schema-ed data).

Note, this will also only fix the case where the type is defined (e.g. string) - if it's untyped, it remains unsupported.

This thus partially fixes https://github.com/OpenAPITools/openapi-generator/issues/318

An example of something this will support is already defined in the sample API:

  additionalPropertiesObject:
    description: An additionalPropertiesObject
    type: object
    additionalProperties:
      type: string