feat: conditionally send aibridge actor headers (#21643)

Also passes along the authenticated username as actor metadata.

Closes https://github.com/coder/aibridge/issues/135
Depends on https://github.com/coder/aibridge/pull/142

**Replace aibridge tag with merge commit once
https://github.com/coder/aibridge/pull/142 lands.**

---------

Signed-off-by: Danny Kopping <danny@coder.com>

docs/reference/api/general.md

@@ -200,6 +200,7 @@ curl -X GET http://coder-server:8080/api/v2/deployment/config \
         },
         "rate_limit": 0,
         "retention": 0,
+        "send_actor_headers": true,
         "structured_logging": true
       }
     },