Add --launcher-msg option to pass to desktop app

Summary:
Allows the launcher to provide messages to the user. Currently, in the form
of the red notification bar at the bottom. This is just meant as a temporary measure
during the alpha to have some clearly noticeable way of getting the user's attention.

I consider removing this a blocker for the release as this mechanism is not well
suited for this in many ways.

The current use case for this is providing a warning if a cached version is used
instead of the requested one, e.g. "Could not fetch requested Flipper version 'deadbeef', using cached version instead."

Reviewed By: jknoxville, priteshrnandgaonkar

Differential Revision: D14073687

fbshipit-source-id: 85630347027063103315eeb1286731fe2478e261

static/index.js

@@ -48,6 +48,11 @@ const argv = yargs
     describe: 'Toggle delegating to the update launcher on startup.',
     type: 'boolean',
   })
+  .option('launcher-msg', {
+    describe:
+      '[Internal] Used to provide a user message from the launcher to the user.',
+    type: 'string',
+  })
   .version(global.__VERSION__)
   .help().argv;