Fix array model type to also work with primitive type

Description

Currently, arrayModelType only works with non-primitive types. It should also work with primitive types.

Swagger-codegen version

3.0.0

Swagger declaration file content or url

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

When using {{{arrayModelType}}} in a template file, I expect something like f64, but I get an empty string.

Command line used for generation

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

Steps to reproduce

Use a primitive type in the items section of an array schema.

Suggest a fix/enhancement

The problem lies on line 1111 of DefaultCodegen.java:
https://github.com/swagger-api/swagger-codegen/blob/d300ebf34411d0e7f42f1c44ab6bd7584443089d/modules/swagger-codegen/src/main/java/io/swagger/codegen/DefaultCodegen.java#L1111

Here the complexType of the array property is used, and as we can see at line 1540, it is obviously not set when we have a primitive type:
https://github.com/swagger-api/swagger-codegen/blob/d300ebf34411d0e7f42f1c44ab6bd7584443089d/modules/swagger-codegen/src/main/java/io/swagger/codegen/DefaultCodegen.java#L1540-L1544

A proposed solution is to use .items.baseType instead of .complexType, so that line 1111 becomes:

codegenModel.arrayModelType = fromProperty(name, schema).items.baseType;

Read DefaultCodegen.java around lines 1111 and 1540-1544, then reproduce the issue with the provided array schema and Rust generation command. Update the array model type handling so the primitive item type is exposed, and verify that the template receives a value such as f64 instead of an empty string.