admin: expose init manager state in /server_info (#5076)

Adds init states to ServerInfo.State to make it easier to determine if
an Envoy instance is stuck initializing.

Fixes #4405

Signed-off-by: Snow Pettersen <snowp@squareup.com>

Mirrored from https://github.com/envoyproxy/envoy @ 755154f96e146ab5e03093ef8a450ff118348d31

envoy/admin/v2alpha/server_info.proto

@@ -17,6 +17,10 @@ message ServerInfo {
     LIVE = 0;
     // Server is draining listeners in response to external health checks failing.
     DRAINING = 1;
+    // Server has not yet completed cluster manager initialization.
+    PRE_INITIALIZING = 2;
+    // Server is running the cluster manager initialization callbacks (e.g., RDS).
+    INITIALIZING = 3;
   }
 
   // State of the server.