[Rust Server] Refactor Mustache templates for request/response body handling

[richardwhiuk]

This PR does a bunch of refactoring of the Rust Server mustache templates to make them more maintainable.

This entails splitting out the request and response body handling from client-operation.mustache into sub-templates.

To make the code more manageable, the Rust Server Codegen now explicitly emits flags for the different types of request bodies which can be consumed, and whether there is a request body at all.

There are very minor changes to the generated code:

• We generate a few more comments in the output, aiding readability and debuggability
• There are some minor whitespace changes for legibility, or due to whitespace expansion on template inclusion.
• When handling a multipart/related request, we now emit the unused elements, in the same fashion as for structured types, like application/json.

PR checklist

• Read the contribution guidelines.
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  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

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  Commit all changed files.
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[wing328]

thanks for the refactoring. let's give it a try