[rust-server] Resolve clippy warnings

Created by: siketyan

This pull request edits rust-server template to resolve all of clippy warnings I confirmed. At the latest master branch, clippy is failing because it is affected by -Dwarnings flags implied in .cargo/config that is also generated by rust-server variant themselves.

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.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

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Would you please review this PR? @frol @farcaller @richardwhiuk @paladinzh