[http-client-csharp] Add preserveUnderscores option to ToIdentifierName and ToVariableName#10062

JoshLove-msft merged 2 commits into

mainmicrosoft/typespec:mainfrom

copilot/http-client-csharp-update-toidentifiernamemicrosoft/typespec:copilot/http-client-csharp-update-toidentifiernameCopy head branch name to clipboard