Rust Client: Add support for collectionFormat="multi"

[isimluk]

Previously, rust-client generator disregarded collectionFormat property of multi-value parameters. Now we support following values in collectionFormat: "", "csv", "multi".

You can review changes this PR yield for fairly sizeable project at https://github.com/CrowdStrike/rusty-falcon/pull/33/files

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• 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/utils/export_docs_generators.sh
  

  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.
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  • hello ✌️ 👋 @frol,(2017/07) @farcaller (2017/08) @richardwhiuk (2019/07) @paladinzh (2020/05)

[wing328]

Tested locally with updated samples and the result is good. Thanks for the contribution.

(next time, please follow step 3 to update the samples so that the CI can verify the change)

[isimluk]

Thank You, @wing328.