Subscription Support

**Is your feature request related to a problem? Please describe.**

I have a requirement for a stitching library that also supports subscriptions.

**Describe the solution you'd like**

I would like to see this library updated to support subscriptions.

**Describe alternatives you've considered**

Braid and Lilo do not support subscriptions. I have started reviewing the code base to try and understand how to effectively implement the subscriptions, though, it looks like it may take considerable work to my untrained eye.

首先检查代码库的 schema-stitching 和 request-orchestration 流程，然后将其与 GraphQL subscriptions 的处理方式进行比较。该 issue 没有指定具体的文件或测试；完成标准是 library 支持跨 stitched services 的 subscriptions，并包含对此行为的验证。

由索引模型根据 Issue 内容生成。