Support Agent Servers on remoting (#42683)

<img width="348" height="359" alt="Screenshot 2025-11-13 at 6 53 39 PM"
src="https://github.com/user-attachments/assets/6fe75796-8ceb-4f98-9d35-005c90417fd4"
/>

Also added support for per-target env vars to Agent Server Extensions

Closes https://github.com/zed-industries/zed/issues/42291

Release Notes:

- Per-target env vars are now supported on Agent Server Extensions
- Agent Server Extensions are now available when doing SSH remoting

---------

Co-authored-by: Lukas Wirth <me@lukaswirth.dev>
Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>

crates/proto/src/proto.rs

@@ -331,6 +331,7 @@ messages!(
     (GetAgentServerCommand, Background),
     (AgentServerCommand, Background),
     (ExternalAgentsUpdated, Background),
+    (ExternalExtensionAgentsUpdated, Background),
     (ExternalAgentLoadingStatusUpdated, Background),
     (NewExternalAgentVersionAvailable, Background),
     (RemoteStarted, Background),
@@ -681,6 +682,7 @@ entity_messages!(
     GitClone,
     GetAgentServerCommand,
     ExternalAgentsUpdated,
+    ExternalExtensionAgentsUpdated,
     ExternalAgentLoadingStatusUpdated,
     NewExternalAgentVersionAvailable,
     GitGetWorktrees,