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/capabilities/default.json

@@ -36,6 +36,8 @@
     "os:default",
     "dialog:default",
     "clipboard-manager:default",
-    "clipboard-manager:allow-write-text"
+    "clipboard-manager:allow-write-text",
+    "updater:default",
+    "process:allow-restart"
   ]
 }