Shutdown command

Summary:
Introduce a 'shutdown' command which effectively kills the process.

Killing the process, for flipper-server, will close down the server and active ws connections.

Reviewed By: passy

Differential Revision: D37209193

fbshipit-source-id: 83303e8209530a41ba870009603cfb383915b7a6

desktop/flipper-common/src/server-types.tsx

@@ -299,6 +299,7 @@ export type FlipperServerCommands = {
   'intern-upload-scribe-logs': (
     messages: {category: string; message: string}[],
   ) => Promise<void>;
+  shutdown: () => Promise<void>;
 };
 
 export type GraphResponse = {