Add configurable cache expiry

[TauCu]

o/

This pull request addresses two key issues with snapshots not being cached:

1. Plugins using Libby, especially on test servers and or those with a large userbase can generate excessive bandwidth for package repositories.
2. It also slows down server configuration and plugin development, especially on slow internet connections.

This pull aims to mitigate both issues while offering additional flexibility if needed.

Changes:

• added cacheDuration(Duration) to Library.Builder
• Snapshots are now cached for 1 day by default

A .timestamp file is used to track the age, rather than relying on file creation dates, which are platform-dependent.

Think of the poor package repositories!