Add Builder to simplify openapi document object construction

A regular C# builder pattern would help a lot in many scenarios.
Building an object now is very complex...

Builder pattern is also the "de facto" in Aspnetcore.. so would make sense to expose a similar api

var openApiDocumentBuilder = new OpenApiDocumentBuilder()
     .WithServers(...)
     .WithOperation(...)
     .WithOperation(...)
     .WithSchema(...);
//... etc


var openApiDocument = openApiDocumentBuilder.Build();
     

此 issue 未指定任何檔案、測試或進入點。先檢視現有的 OpenAPI 文件物件模型與目前的建構模式，然後在實作之前釐清 builder 支援的操作、schemas 與 servers。完成的標準是：達成共識的 builder API 能透過 Build() 建構出等效的 OpenAPI 文件。