feat: Send transactions in envelopes

This matches what the JS SDK does and what the Tracing dev docs
indicates.

Author: rhcarvalho

sentry_sdk/client.py

@@ -330,7 +330,12 @@ def capture_event(
         if session:
             self._update_session_from_event(session, event)
 
-        self.transport.capture_event(event_opt)
+        if event.get("type") == "transaction":
+            # Transactions should go to the /envelope/ endpoint.
+            self.transport.capture_envelope(Envelope(items=[event]))
+        else:
+            # All other events go to the /store/ endpoint.
+            self.transport.capture_event(event_opt)
         return event_id
 
     def capture_session(