[Rust Server] Support multipart/form_data request bodies

[richardwhiuk]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
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@frol, @farcaller, @bjgill

Description of the PR

• Support multipart/form_data in the Rust Server generator
• Add a new test API to test the change.
• Update the examples to match

This has been written, and reviewed by myself, and others at @Metaswitch.

[richardwhiuk]

CI Failure is in the elm code, which should be unrelated (e.g. it's seen in the merge of #2112).

[wing328]

@richardwhiuk right, the CI failure (addressed in the latest master) has nothing to do with this PR.

If you are you free, let's have a quick chat via https://gitter.im (ID: wing328).

[bjgill]

Thanks for the PR - looks good. I've just got a dependency version question and a minor style tweak.