[RUST] chore: Clean up Rust reqwest and model templates

[ranger-ross]

This PR is an attempt to make the templates for the Rust client a bit more maintainable.
Currently, we have all of the function signature is a single line making it very difficult to reason about/update.

The approach here is to use mustache comments (ie. {{! my comment }} ) to annotate parts of the template and break things into multiple lines.

There are no changes when running the ./bin/generate-samples.sh ./bin/configs/*.yaml script so the changes should not affect the generated clients.

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• Run the following to build the project and update samples:

  ./mvnw clean package || exit
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cc: @frol @farcaller @richardwhiuk @paladinzh @jacob-pro

[wing328]

FYI. For situation like this (too many mustache tags in a single line), another way to refactor this is to do it in the generator (java class), such as postProcessOperationsWithModels and store the value in x-pub-params in extensions for example.