feat(updater): add tauri-plugin-updater backend integration

Adds tauri-plugin-updater and tauri-plugin-process crates, the updater
config block in tauri.conf.json (with PLACEHOLDER_PUBLIC_KEY to be
replaced before release), capabilities for updater/process, and the
v2_is_auto_update_eligible command that returns true on macOS and
Linux AppImage installs only.

Cherry-picked from #305 by @afonsojramos.

cc

src-tauri/tauri.conf.json

@@ -36,11 +36,18 @@
       "desktop": {
         "schemes": ["qobuzapp"]
       }
+    },
+    "updater": {
+      "endpoints": [
+        "https://github.com/vicrodh/qbz/releases/latest/download/latest.json"
+      ],
+      "pubkey": "PLACEHOLDER_PUBLIC_KEY"
     }
   },
   "bundle": {
     "active": true,
     "targets": "all",
+    "createUpdaterArtifacts": true,
     "icon": [
       "icons/32x32.png",
       "icons/128x128.png",