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

src/utils/__tests__/processConfig.node.js

@@ -16,6 +16,7 @@ test('config is decoded from env', () => {
     disabledPlugins: ['pluginA', 'pluginB', 'pluginC'],
     pluginPaths: ['/a/path', 'b/path'],
     lastWindowPosition: {x: 4, y: 8, width: 15, height: 16},
+    launcherMsg: 'wubba lubba dub dub',
     updaterEnabled: false,
     screenCapturePath: '/my/screenshot/path',
     launcherEnabled: false,
@@ -25,6 +26,7 @@ test('config is decoded from env', () => {
     disabledPlugins: new Set(['pluginA', 'pluginB', 'pluginC']),
     pluginPaths: ['/a/path', 'b/path'],
     lastWindowPosition: {x: 4, y: 8, width: 15, height: 16},
+    launcherMsg: 'wubba lubba dub dub',
     updaterEnabled: false,
     screenCapturePath: '/my/screenshot/path',
     launcherEnabled: false,
@@ -38,6 +40,7 @@ test('config is decoded from env with defaults', () => {
     disabledPlugins: new Set([]),
     pluginPaths: [],
     lastWindowPosition: undefined,
+    launcherMsg: undefined,
     updaterEnabled: true,
     screenCapturePath: undefined,
     launcherEnabled: true,