Summary

Use Literal type to better type the accepted values for encoding in WebSocketEndpoint

The documentation already states that it only accepts 'text', 'bytes' or 'json' so this only enforces it at typing level

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.