[rust] Fix for allOf multi model with only metadata fields

[ranger-ross]

This PR fixes one of the bugs reported in #20500.

The following schema will generate models with a Vec with the generic parameter causing compiler errors.

components:
  schemas:
    Order:
      type: object
      properties:
        id:
          type: integer
          format: int64
        foo:
          allOf:
            - $ref: '#/components/schemas/existing_tags_array'
            - description: 'this is foo'

Because the second parameter simply metadata that is not important to code generation we can simply ignore it.
NOTE: This fix does not add full support for allOf with multiple non-model types.

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cc: @frol @farcaller @richardwhiuk @paladinzh @jacob-pro
cc @OpenAPITools/generator-core-team @wing328 since this fix may change the behavior of other clients

[wing328]

thanks for the PR

would it be better to put this method in the model util class as static method?