Add JS API for Clearing GraphQL cache

[ianpward]

In the context of the customer Yuka we encountered an issue where (according to Nikola)

GraphQL never responds to the query and most likely the 502s they are reporting are a result of some gateway timeout

To remedy this it was necessary for us to clear the ROS-level Realm cache, which is cumbersome and requires as well the customer to issue a service request.

Can we please add a convenience api to allow either Realm SE's or even the customer to remedy this without involving engineering. Proposed is a schema as follows:
DELETE /graphql/schema/:path

We can then update the documentation here to reflect this:
https://github.com/realm/realm-graphql-service#endpoints