Support use x-* in ref

[krissss]

@OA\Schema(
     schema="String",
     @OA\Property(property="value", type="string"),
     x={
        "custom-key": @OA\Schema(
            @OA\Property(property="value", type="string"),
        )
    }
)

The above annotation will generate x-custom-key in the OpenAPI document.

However, we need to get ref of x-custom-key with $openapi->ref('#/components/schemas/String/x/custom-key') (note that here we use x/custom-key instead of x-custom-key).

This is not conducive to writing ref in the code ourselves. For example,

@OA\Post(
    path="/api/~/endpoint",
    @OA\Response(
        response="default",
        description="A response",
        @OA\JsonContent(ref="#/components/schemas/String/x/custom-key")
    )
)

This will generate the following incorrect documentation when using $openapi->toYaml()(use x/custom-key)

...
            application/json:
              schema:
                $ref: '#/components/schemas/String/x/custom-key'
...

Therefore, we have to write it in the following way (use x-custom-key)

@OA\Post(
    path="/api/~/endpoint",
    @OA\Response(
        response="default",
        description="A response",
        @OA\JsonContent(ref="#/components/schemas/String/x-custom-key")
    )
)

However, if it is written directly in the above way, the validation will fail.

This PR is to fix this issue to support the use of ref x-*.

[DerManoMann]

Interesting - I didn't even know you can nest complex structures in x. Somehow I always assumed it was a flat map...

[DerManoMann]

Nice!

[DerManoMann]

Thanks @krissss