GraphQL playground equivalent for JSON protocol

[SevInf]

For GraphQL, binary query engine can expose GraphQL playground that allows us to have autocomplete for the queries and browse the schema. We don't have an equivalent for JSON protocol. With the possibility of GraphQL protocol going away in the future, it would be nice to have an alternative. Even without potential removal, we still might want to have convenient way to debug JSON-specific issues.

I suggest --enable-playground flag, if used together with JSON protocol would expose an alternative interface that would have an equivalent functionality for GraphQL Playground.