[rust-axum] Add support for multiple response types and streaming responses

[jacob-mink-1996]

This change enables two related features in the rust-axum server generator

1. text/event-stream responses
2. multi-part responses (e.g. 200: application/json & text/event-stream)

The core of the feature is to iterate over the content types rather than taking just the first, then build up a new list in the vendor extensions that includes the appropriate information. I took the approach of still keeping the templates simple and tried to compute types in the Java code as much as possible - we could of course go the other way and expand the type within the template, but it got pretty gnarly.

@linxGnu - I adopted a bit of a hybrid approach to the typing we discussed since this isn't just a return type change, it changes the enums. I went ahead and used a

Pin<Box<dyn Stream<Item = Result<..., Box<dyn Error>>>>>

(along with appropriate Sends/Syncs/'static annotations). This allowed me to shove it in an enum, and it appears to play nice with other methods that return an impl Stream or impl TryStream, both of which can be coerced into the pin-box using the .boxed() method from StreamExt.

Finally, the templates are updated to support the new code gen. I have validated this two ways:

1. I wrote some unit tests that verify that the right types and names are present
2. I ran this generator on the real project I'm working on that uses this and have been able to refactor my code based on this new support nicely.

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  fixes [REQ] [rust-axum] support text/event-stream #21967
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[linxGnu]

@jacob-mink-1996 just fyi, I will have a look this weekend