Added reading and writing of a state-cache

[srogmann]

This PR contains an exemplary implementation for storing the computed states of a system-prompt on disk.

I wrote this implementation a few weeks ago, so it may not be mergeable directly.

[mukel]

Hey @srogmann, this this awesome!
I rebased an played with it, there are a some rough edges.

• The cached prompt must match exactly the given prompt, to improve usability is should start from the largest prefix that matches.
• Needs some usability polish, it's not clear (no docs) how to cache a prompt and how to use it.

This feature is a must have, and I'd really like caching to be composable, so it can be easily and transparently turned on for e.g. completions API. I've discussed with a colleague how to do caching on disk and we have some ideas, note that KV caches can be also quantized to save memory and disk space.

I'm busy this week, but this is a good start!