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