Add support for "graphql-transport-ws" subscription subprotocol

[lorenzocesconetto]

🚀 Feature Request

Description

• The subprotocol is currently hardcoded to graphql-ws.
• But graphql-transport-ws is the newest and recommended subprotocol.

Implementation details

• Add a subprotocol argument to WebSocketEndpoint.

Potential caveats

• Keeping backwards compatibility, not breaking lib's API.

Acceptance criteria

• Can subscribe to servers using graphql-transport-ws subprotocol.

Additional context and visual reference

[barbieri]

I don't have much time to work on this right now, can you send a PR? Just be sure to test it with pytest and pre-commit scripts before opening it, ok?