Add "useSingleRequestParameter" option to Rust client generator

[wing328]

• Add "useSingleRequestParameter" option to Rust client generator so that the method parameters will be grouped in a single struct
• Tested locally and the result is good.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

cc @frol (2017/07) @farcaller (2017/08) @bjgill (2017/12) @richardwhiuk (2019/07) @paladinzh (2020/05)

[wing328]

OK. Will update those.

[wing328]

Submitted #6381

cc @pbeyeler

[pbeyeler]

@wing328 Thank you!

[wing328]

#6381 merged into master. Thanks for reporting the issue and suggesting the fix.

[wing328]

@pbeyeler I've filed #6453 to export the struct. Please take a look when you've time.

[pbeyeler]

@wing328 Looks good to me.

However, I used the Stripe API spec (https://github.com/stripe/openapi/raw/master/openapi/spec3.yaml) for testing, which is quite complex. Trying to compile the generated code resulted in 1256 errors (Rust 1.43.0). This also happens with v4.3.1. Most of these errors boil down to the same 4 or 5 causes. If you have time, I'd really appreciate if you could fix these errors as well. Thank you!