[Rust-Axum][Breaking Change] Improve the oneOf model generator

[Victoria-Casasampere-BeeTheData]

This PR aims to replace the current oneOf and discriminator implementation for the rust-axum generator to one that is usable.

The idea is to generate an enum for oneOf models, with a variant for each specified model. It uses untagged when the discriminator is not specified, and tag = "discriminator" when it is.
This implementation however is not the default present in serde, as it lacks serialization for the discriminator field, a field that has to be specified in all oneOf variants per OAS, and will fail to serialize when present in the generated structure. The custom implementation allows for the field to be present, and still be able to serialize to the adequate model.
Another benefit of the custom serialization is that the variant structure can be serialized directly without the need to convert to the original enum (an option which is still available), which has its benefits when the models have further use outside the API (such as WS messages), and facilitates the use of the frunk crate.

TODO:

• Figure out why ./bin/generate-samples.sh bin/configs/manual/rust-axum-* freezes, but manually generating each model still works.
• Fix the validator implementation of primitive type enum variants.
• Move model modifications from postProcessOperationsWithModels to its own method.
• Support discriminators specified in non oneOf objects...?
• Add a test case for tagged discriminators in oneOf objects.

Fixes #20101

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[Victoria-Casasampere-BeeTheData]

This was annoying to me, it broke string formatting, which made debugging slightly harder.

[wing328]

👌

thanks for the fix.

cc @OpenAPITools/generator-core-team

[Victoria-Casasampere-BeeTheData]

@linxGnu @wing328
Ready for review

[wing328]

thanks for the PR

would it be possible to add some oneOf model tests (serialization, deserialization) similar to what we've done in the java client, e.g. https://github.com/OpenAPITools/openapi-generator/blob/master/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/JSONTest.java#L410 ?

[Victoria-Casasampere-BeeTheData]

I will have a look at implementing a test case

[Victoria-Casasampere-BeeTheData]

@wing328 Tests for tagged and untagged enums have been implemented and are passing on the CI. Ready for review.