[rust-client] Ability to specify path of output

Currently output is always generated as src/{apis,models} as done by:

https://github.com/OpenAPITools/openapi-generator/blob/16552755bffa4f1a4d7e1afc67cf7e97dc1cd1f4/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/RustClientCodegen.java#L85-L86

In our case we run with a very limited generator (we only want the models).

.openapi-generator-ignore:

**
.openapi-generator/*
!src/models/*.rs

What we end up doing is a little odd:

mkdir -p generated/rust
rm -rf ./src/api/models
docker run --user $(id -u) --rm -v "${PWD}:/local" -w /local/generated/rust openapitools/openapi-generator-cli:v7.12.0 generate -i /local/api/kiburi.v0.yml -g rust --config /local/.openapi-generator/rust/config.yaml
mv ./generated/rust/src/models ./src/api/model

Ideally it would be nicer if we could directly write the models etc to a subfolder inside our rust crate and not require manual modification.

I can imagine an additionalProperty: asModule: true or something like this.

Start with modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/RustClientCodegen.java at the referenced lines, then inspect the Rust generator configuration and the docker generate command shown in the issue. Define how an additional property or equivalent configuration should select an output subfolder, and verify that models can be generated directly into the Rust crate without the manual move.