[BUG] [Rust Server] yaml self reference code compile error.

Created by: paladinzh

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
What's the version of OpenAPI Generator used?
Have you search for related issues/PRs?
What's the actual output vs expected output?
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Description

Using self reference in yaml, the generated Rust code compile error.

openapi-generator version

3.0.1

OpenAPI declaration file content or url

Error:
  type: object
  required:
    - code
    - retryable
    - message
  properties:
    code:
      type: integer
    retryable:
      type: boolean
    message:
      type: string
      example: NotFound
    innerError:
      $ref: "#/components/schemas/Error"

Command line used for generation

bin/rust-server-petstore.sh

Steps to reproduce

add above yaml code to modules/openapi-generator/src/test/resources/3_0/rust-server/openapi-v3.yaml, then comile the generated code.

Related issues/PRs

Suggest a fix

Add isSelfReference to check self reference var, and If true add Box<>.