[Rust] Add support for array schemas in Rust template

Description

Currently, the Rust template will generate an empty struct when trying to generate a model from an array schema. This is incorrect and undesirable. Instead, we should generate a 1-item tuple like struct Test(Vec<f64>);

Swagger-codegen version

3.0.0

Swagger declaration file content or url

components:
  schemas:
    Test:
      type: array
      items:
        type: number
        format: double

This currently generates struct Test { }.

Command line used for generation

java -jar swagger-codegen/modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate \
   -i apis/gdax/openapi.yaml \
   -l rust \
   -o apis/gdax/generated_api

Steps to reproduce

Try to generate a model from an array schema for Rust.

Suggest a fix/enhancement

A proposed fix is to use vendorExtensions.x-is-array-model in the rust template and to generate different code depending on it.

Start with the Rust template used by the -l rust generator and inspect how vendorExtensions.x-is-array-model is handled. Reproduce the issue with the supplied array schema and generation command; done when Test is generated as a one-item tuple such as struct Test(Vec); rather than an empty struct.