Auth signing fixes for GET requests

Fixed a small issue causing some authenticated GET requests to return a "not logged in" error.

WebSocket & Signing Fixes

• Fix(#6) websocket authentication for subaccounts.
• Fix(#8) topic matching and unsubscribing for websockets.
• Fix(#10) sign error for specific request types.

Fix getPositions suffix

The following endpoints were unnecessarily suffixed with ? when an optional parameter was missing, causing 401 errors for signed requests:

• getOrderbook()
• getPositions()
• getDepositAddress()
• getOpenOrders()
• getQuoteStatus()
• getMarketInfo()

FTX.us REST API & websocket support

This release improves types & adds support for the FTX.us exchange REST APIs and WebSockets.

See the readme for more details: https://github.com/tiagosiebler/ftx-api#ftxus