[Feature] Centralized Automated Update Checker

[echo6ix]

Is your feature request related to a problem? Please describe.
Currently, there is no streamlined way to check for updates across multiple components of Stash, such as plugins, metadata scrapers, and the application itself. Users have to manually verify and update each, which is prone to oversight.

For example, what I just encountered was using outdated CommunityScrapers to submit incomplete content to a stash-box because I had forgotten to periodically check for CommunityScraper updates manually.

Describe the solution you'd like
A centralized automated update checker that:

• Periodically scans for updates across any components that require possible updating (currently the plugins, metadata scrapers, and the application itself).
• Fetches metadata about each update, such as:
  • "New versions of scrapers are available for {package_names[]}"
  • "New versions of scrapers are available for {plugin_names[]}"
  • "New version of Stash available"
• Provides integration points for notifying other systems in Stash, such as a user notification interface that lives on the main nav bar

Describe alternatives you've considered
Creating separate automated update checkers for plugins, scrapers, and the app, but this approach would lack centralization and maybe increase complexity?

Additional context
This feature would work in tandem with a user notification system #5626 , that could reside as a conventional notification bell in the main nav bar, to inform users when updates are available.