Feat/rust reqwest shallow object query param

[whirm]

Recreating #21199 as it turns out one can't reopen a PR after force-pushing to its branch.

• Add support for flat object query parameters.
• Expand objectQueryParam.yaml to cover both required and optional variants.

Note that in some cases it changes the existing behavior (serialize object to JSON string) to flattening the object. Which AFAIU is what the 3.0 specs ("schema vs content" subsection) indicate as the correct behavior. I've double checked by running such a spec with swagger UI and inspecting the generated queries.

PR checklist

• [x] Read the contribution guidelines.
• [x] 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.
• [x] 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 Git BASH) 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.
• [x] 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)
• [x] If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@frol, @farcaller, @richardwhiuk, @paladinzh, @jacob-pro, @linxGnu Ready to review!

[whirm]

@linxGnu, @wing328 Excuse the switcharoo.

As far as I can tell, no changes are required on the codegen. I left the unreachable! Value::Object variant on the match as the user will never see it and it makes for a much clearer error message if someone accidentally breaks it.