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.

Start by reviewing the codebase's schema-stitching and request-orchestration flow, then compare it with how GraphQL subscriptions are handled. The issue does not name specific files or tests; done would require the library to support subscriptions across the stitched services and include validation of that behavior.

索引モデルが issue の本文から書いたものです。