Add a way to create schemas

Hi,

The library allows to create paths, but apparently doesn't provide any way to add schemas.

Doing something like:
`$objOpenapi->components->schemas['foo'] = new Schema([
'type' => Type::INTEGER,
]);`

gives a warning and the element is not created:
PHP Notice: Indirect modification of overloaded property cebe\openapi\spec\Components::$schemas has no effect in G:\test.php on line 119

Am I missing something ?

If not, it would be nice if we could create dynamically elements using this library.

Thank in advance

首先复现 issue 中的示例，并检查 cebe\openapi\spec\Components 如何处理重载的 schemas 属性。确定动态添加 schema 的受支持方式；当能够创建并添加 schema 且不会出现报告中的 PHP notice 时，即视为完成。

由索引模型根据 Issue 内容生成。