Code Exporter does not work with subscriptions.

### Version Information

Server Version: v2.15.1

CLI Version (for CLI related issue):

### Environment
OSS

### What is the current behavior?

Currently, the code exporter seems to only work for queries and mutations. For subscriptions its says "The query is invalid.
The generated code will appear here once the errors in the query editor are resolved."

### What is the expected behaviour?
Code exporter generates code snippet for subscriptions

### How to reproduce the issue?
Run the code exporter with any subscription or streaming subscription.

### Screenshots or Screencast

### Please provide any traces or logs that could help here.

NA

### Any possible solutions/workarounds you're aware of?

NA

### Keywords

NA

Start at the Code Exporter entry point and reproduce the issue with a query, mutation, subscription, and streaming subscription. Compare how subscriptions are handled in the query editor, and consider the issue done when the exporter generates a code snippet for subscription operations without reporting that the query is invalid.

Written by the indexing model from the issue text.