rust: support repr(int) enum

Created by: yjh0502

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description

It adds support for integer-typed enums to rust client binding. Here's example schema:

openapi: 3.0.1
info:
  title: test
  version: v1
servers:
  - url: http://127.0.0.1:3000/
paths:
  /testapi:
    post:
      responses:
        '200':
          $ref: '#/components/schemas/Test'
components:
  schemas:
    Test:
      type: integer
      enum:
        - 101
        - 102

output for the schema is as follows:

 #[repr(i64)]
  #[derive(Clone, Copy, Debug, Eq, PartialEq, Ord, PartialOrd, Hash, Serialize_repr, Deserialize_repr)]
  pub enum Test {
      _101 = 101,
      _102 = 102,

  }

  impl ToString for Test {
      fn to_string(&self) -> String {
          match self {
              Self::_101 => String::from("101"),
              Self::_102 => String::from("102"),
          }
      }
  }

Current implementation assumes all enums are string-represented so the schema above fails to decode int-encoded enums. This PR adds support for int-typed enums with [repr(i64)].

cc @paladinzh