Expose a middleware to output Open API description for a certain OData service

Maybe we can have an extension middleware to expose the OpenAPI description for a certain OData service.

For example:

public class ODataOpenApiMiddleware
{
    private readonly RequestDelegate next;

    public ODataOpenApiMiddleware(RequestDelegate next)
    {
        this.next = next;
    }

    public async Task Invoke(HttpContext context)
    {
        // implementation here
    }
}

Extension method:

public static IApplicationBuilder UseODataOpenApi(this IApplicationBuilder app)
{
    return app.UseMiddleware<ODataOpenApiMiddleware>();
}

Configure the Middleware:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{

       app.UseODataOpenApi();

       app.UseRouting();

       app.UseEndpoints(...);
}

从此 issue 中提议的 ODataOpenApiMiddleware、其 Invoke 方法、UseODataOpenApi 扩展以及 Configure 示例开始。明确中间件应如何选择某个 OData 服务，以及哪些响应和路由行为定义了完成条件；未确定任何实现文件或测试。