This release includes bug fixes, improvements and documentation updates

• introduce SubscribeRaw and wrap all subscriptions map access with lock to avoid race condition (#4) (thank @digitalcrab)
• allow clients to receive all GraphQL errors from server (#8) (thank @totalys)
• support raw JSON field with json.RawMessage type (#14) (thank @kwapik)
• expose WebsocketHandler and custom WebSocket client documentation (#15)
• misc: automation test with Github Action