[Rust Server] Use swagger/multipart_related support

[richardwhiuk]

Use functions from the https://github.com/Metaswitch/swagger-rs crate for multipart/related helper functions, rather than including the logic in every generated crate.

This updates the generated code to use functions from the swagger-rs crate, rather than having hand rolled code in each crate. Code in the support crate is easier to test, update and bug fix, rather than generated code which requires each generated crate to be updated.

Support was added in 5.0 - the generated code already uses the swagger-rs 6

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