[rust-axum] Fix polymorphic type discriminators

[KevinLiddle]

This PR attempts to address 2 issues I am having with the rust-axum code generation for polymorphic types:

1. The generated field for a discriminator property is not properly given a serde rename attribute with the camelCase version of the attribute name. This is inconsistent with other generated fields.
2. The generated enum for a polymorphic type that uses a discriminator mapping does not do anything with the mapping names. This results in generated clients in other languages (e.g. TypeScript) passing the mapping keys, but the rust-axum generated server only expects the mapped type name.

Please review updates to tests and samples for examples of my use cases.

cc @linxGnu

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[linxGnu]

LGTM. Thank you for making the fix

[linxGnu]

@KevinLiddle

Test failing because:

Looks like samples/server/petstore/rust-axum/output/rust-axum-oneof/tests/oneof_with_discriminator.rs has been modified as its SHA256 `b2093528aac971193f2863a70f46eea45cf8bda79120b133a614599e80d8b46d` is not the same as the one in the record: 2d4f5a069fdcb3057bb078d5e75b3de63cd477b97725e457079df24bd2c30600

Could you please update this line with new checksum: b2093528aac971193f2863a70f46eea45cf8bda79120b133a614599e80d8b46d

[KevinLiddle]

@linxGnu updated. Thanks for the quick review!

[linxGnu]

@wing328 PTAL. I think it's good to merge