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();
     

该 issue 没有指定文件、测试或入口点。首先检查现有的 OpenAPI 文档对象模型和当前的构造模式，然后在实现之前明确 builder 支持的操作、schemas 和 servers。完成的标准是：达成一致的 builder API 可以通过 Build() 构造出等价的 OpenAPI 文档。