Fix typing of `JetStreamContext.publish`

[rijenkii]

Argument headers has type Optional[Dict[Unknown, Unknown]], which makes pyright scream and shout in strict mode:

Type of "publish" is partially unknown
  Type of "publish" is "(subject: str, payload: bytes = b'', timeout: float | None = None, stream: str | None = None, headers: Dict[Unknown, Unknown] | None = None) -> Coroutine[Any, Any, PubAck]"PylancereportUnknownMemberType