[Rust] reqwest shallow object query param

[whirm]

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

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

[linxGnu]

IMHO, having unimplemented here may cause unexpected panic from user point of view. How about auto fallback to parse_deep_object:

serde_json::Value::Object(_) => {
   return parse_deep_object("", value);
}

or just don't handle it, let it fallback to _ =>