[Rust] Add support for native-tls and rustls-tls Cargo features for the reqwest library

[JMLX42]

Implements #21924

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 || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  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.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR solves a reported issue, reference it using GitHub's linking syntax (e.g., having "fixes #123" present in the PR description)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[JMLX42]

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

[wing328]

thanks for the PR

CI tests failed: https://github.com/OpenAPITools/openapi-generator/actions/runs/17565976588/job/49899025424?pr=21925

please take a look when you've time.

[dsteeley]

To mirror the behaviour of reqwest would it be prudent to expose all three *-tls features and follow reqwest's default behaviour?
I think that setting the default feature to default-tls or providing no default feature override would be preferable to setting native-tls to the default.

seanmonstar/reqwest#2025 is relevant here.

[JMLX42]

To mirror the behaviour of reqwest would it be prudent to expose all three *-tls features and follow reqwest's default behaviour?

I agree.

I think that setting the default feature to default-tls or providing no default feature override would be preferable to setting native-tls to the default.

I agree. But wouldn't that be a breaking change, so a different PR?

[JMLX42]

CI tests failed: https://github.com/OpenAPITools/openapi-generator/actions/runs/17565976588/job/49899025424?pr=21925

please take a look when you've time.

@wing328 cargo update once_cell did the trick.

providing no default feature override would be preferable to setting native-tls to the default.

IMO it is. default-tls is non deterministic because platform dependant: I just hit this issue when building with cross for Android. No default feature lets the user chose between the backend.

Update: separate issue here #21933