[rust] fix unusable oneOf generation

[ctrlaltf24]

When generating the combined oneOf struct ensure the struct can form a valid request.

Reproduction steps:

• java -jar ./modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -g rust -i ./openapi.yaml

openapi: 3.0.1

info:
  version: "1"
  title: rust one-of fix
paths: {}
components:
  schemas:
    Foo:
      x-one-of-name: Foo
      oneOf:
        - type: object
          required:
            - field_one
          properties:
            field_one:
              type: string
            field_two:
              type: string
        - type: object
          required:
            - field_three
          properties:
            field_three:
              type: number
    Bar:
      type: object
      required:
        - two
      properties:
        one:
          type: string
        two:
          type: string

• Observe ./src/models/foo.rs

Expected behavior: Able to fill in struct fields in such a way that creates a valid request
Actual behavior: Impossible to fill the struct Foo to make it a valid schema (as BOTH required fields (field_one and field_three) are required to be filled in rust, yet the openapi schema only allows one or the other to be set.

Related issue: #9497

Ideally oneOf would be represented as an enum. Presently the oneOf implementation is unusable (as described by @cortopy), this PR would at least makes it usable, but requires the rust api consumer to know what a valid schema object is (aka which optional types to populate to form a valid response).

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[mattoni]

It'd be great to get this merged in - currently unable to use my open api spec with rust.

[ctrlaltf24]

superseded by #13970