feat: return hostname suffix on AgentConnectionInfo (#17334)

Adds the Hostname Suffix to `AgentConnectionInfo` --- the VPN provider will use it to control the suffix for DNS hostnames.

part of: #16828

docs/reference/api/agents.md

@@ -698,7 +698,8 @@ curl -X GET http://coder-server:8080/api/v2/workspaceagents/{workspaceagent}/con
       }
     }
   },
-  "disable_direct_connections": true
+  "disable_direct_connections": true,
+  "hostname_suffix": "string"
 }
 ```