[REQ][GO] Optionally use public fields for request fields

Is your feature request related to a problem? Please describe.

Request structs in generated golang client code contain private members, which makes testing them directly more difficult. It forces either reflection or using a mock server and parsing JSON.

Describe the solution you'd like

It would be helpful if the field names could be set to public instead, controlled by a configuration option to avoid a breaking change.

Describe alternatives you've considered

Additional context

I believe this change just involves the paramName field used in the api mustache template, and so new logic could be added to AbstractGoCodegen to control how the paramName is generated.

If I'm overlooking anything in the existing codebase, or if people have other strategies for testing their requests, I would definitely love to hear your input!

Start with AbstractGoCodegen and the api mustache template, focusing on how the paramName field is generated. Trace how generator configuration options are defined and passed into the template. Done means an option can select public request fields without changing the default, and generated Go request structs reflect that choice.