Add WebSocket functionality to httpx

[codecov[bot]]

Codecov Report

Attention: 26 lines in your changes are missing coverage. Please review.

Comparison is base (ac4221c) 90.88% compared to head (366930e) 89.93%.

Files	Patch %	Lines
httpx/websocket.go	71.73%	23 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #108 +/- ## ========================================== - Coverage 90.88% 89.93% -0.95% ========================================== Files 38 39 +1 Lines 1766 1858 +92 ========================================== + Hits 1605 1671 +66 - Misses 127 150 +23 - Partials 34 37 +3 ```

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