[Rust] Enhancements to Rust generator

Description

Here is a list of enhancements I can think of for the Rust client generator:

• Add "docstring" to methods in API and model files for documenting the usage
• Add support for form parameters
• Add support for file upload (type: file)
• Add validation for parameters and model properties
• Make timeout (http) configurable
• Implement 'object' type conversion (https://github.com/swagger-api/swagger-codegen/pull/6846)

If anyone has any feedback/suggestion, please reply to let us know.

Swagger-codegen version

Latest master

Command line used for generation

mvn clean install
java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate  -t modules/swagger-codegen/src/main/resources/rust -i modules/swagger-codegen/src/test/resources/2_0/petstore.yaml -l rust -o samples/client/petstore/rust

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/pull/6105

If anyone wants to work on these enhancements, please reply to let us know and we'll show you some good starting points.

Start with the Rust generator templates under modules/swagger-codegen/src/main/resources/rust and the petstore input at modules/swagger-codegen/src/test/resources/2_0/petstore.yaml. Run the listed Maven build and generation command, then review the unchecked enhancements individually alongside related PR 6105. Done would require a narrowly scoped enhancement with corresponding generated output or tests.