feat: add optional headers support to WebSocket connection

Introduce an optional `ws_headers` parameter to the `connect` method in
`VoiceAgentClient`. This allows users to pass custom headers when
establishing the WebSocket connection to the Speechmatics API.

Author: sam-s10s

sdk/voice/speechmatics/voice/_client.py

@@ -472,12 +472,15 @@ def _prepare_config(
     # LIFECYCLE METHODS
     # ============================================================================
 
-    async def connect(self) -> None:
+    async def connect(self, ws_headers: Optional[dict] = None) -> None:
         """Connect to the Speechmatics API.
 
         Establishes WebSocket connection and starts the transcription session.
         This must be called before sending audio.
 
+        Args:
+            ws_headers: Optional headers to pass to the WebSocket connection.
+
         Raises:
             Exception: If connection fails.
 
@@ -521,6 +524,7 @@ async def connect(self) -> None:
             await self.start_session(
                 transcription_config=self._transcription_config,
                 audio_format=self._audio_format,
+                ws_headers=ws_headers,
             )
             self._is_connected = True
             self._start_metrics_task()