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.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.6.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

Rust Technical Committee: @frol @farcaller @paladinzh @jacob-pro