refactor: Split WebSocket support into separate sync/async implementations

[dolfies]

This is a continuation of #206, which went stale after I got quite busy. Addresses #202 WebSocket requests (fixes #202, fixes #270).

• Separates async support into AsyncWebSocket
• Removes Session.ws_connect() and allows initialization via WebSocket() constructor directly
  • This is because WebSockets need their own curl handle, as making a request will break the CurlOpt.CONNECT_ONLY value, so the session becomes useless if a WebSocket is created from it
  • This is not an issue with AsyncSession.connect() as it already maintains multiple curl handles, though a curl handle is unusable after a WebSocket uses it anyway
• Changes WebSocket API to more closely match websockets
• Changes AsyncWebSocket API to more closely match aiohttp
  • Removes sync-style callback handlers
  • Adds send/recv_str/json helper methods
• Adds on_data callback to WebSocket
• Adds autoclose and skip_utf8_validation parameters, and close_code and close_reason attributes
• Adds iteration support

[lexiforest]

Looks good, please let me know when the PR is ready for review.

[dolfies]

Looks good, please let me know when the PR is ready for review.

Should be ready!

[lexiforest]

Could you please update the websocket section on README and the websocket examples in examples folder?

[dolfies]

Done

[lexiforest]

LGTM, I will have a few trivial API adjustment and merge.