feat(rust): implement from trait on enum variants

[ctrlaltf24]

Helpful Quality of Life to reduce verbosity.

Before PR:

let apple_fruit = Fruit::Apple(Box::new(Apple {
  seeds: 5
}));

After PR additional option is added:

let apple_fruit: Fruit = Apple {
  seeds: 5
}.into();

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 7.1.0 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.

@frol @farcaller @richardwhiuk @paladinzh @jacob-pro @wing328

[DDtKey]

Note that {{#oneOf}} {{{.}}} {{/oneOf}} isn't enough here.
Even against current code - there is a case with mappedModels, but actual issue is that current implementation in master have several issues, including wrong imports.

You can take a look at #17915
I think this feature should be introduced only after the fixes

[ctrlaltf24]

Sending into draft to focus rust effort on #17915

[ctrlaltf24]

Closing PR, don't have time to work on it right now, someone else is free to pick up.